The Bellflowers (Campanula, Campanulaceae) are coming into their own right now. Because there are so many of them (18 spp. in Stace, and many more in specialist collections) they can be a bit daunting. 
The two commonest natives are C. rotundifolia (Harebell in England, Bluebell in Scotland, left) of acid grassland and C. glomerata (Clustered Bellflower, right) of chalk and limestone grassland.
The 2 most likely to be seen on walls and pavements in town are “posh” and “port”: C. poscharskyana (Trailing, left) has pale blue star-shaped flowers and larger, pale green leaves; C. portenschlagiana (Adria, right) has dark blue bell-shaped flowers and small dark green leaves
The first question has to do with the sepals. Is the calyx simple (with 5 lobes, left) or complicated (right, 5 calyx lobes alternating with 5 reflexed, sepal-like appendages, right) ?
The 2 spp. with a complex, 10-part calyx are easy to tell apart: fat bells or slim foxgloves ? C. medium (Canterbury-bells, left) has 5 stigma lobes and cuneate leaves; C. alliariifolia (Cornish Bellflower, right) has 3 stigmas and lower leaves cordate. Petal colours vary a lot.
The main part of the key starts with a question that is really difficult to understand on first acquaintance. It has to do with the capsule. This is the swollen bit underneath the calyx.
You could easily go wrong here, so seek out the oldest flowers on the plant. You need to locate the ‘capsule pores’. When ripe, these are the openings that release the seeds. The pores are either at the base of the capsule (left) or in the upper (apical) half (right).
Let’s start with the species that have the pores at the top pf the capsule (i.e. in the apical half). If you don’t have any mature fruits, this is going to be a tricky decision. But not one to get wrong if you can possibly help it.
Look carefully at the edge of the calyx lobes (x10). If they are serrate, you have Campanula lactiflora (Milky Bellflower, left, the commonest cultivar of which is the beautiful, pale ‘Loddon Anna’). If not, it’s something else (right).
An easy one now, based on flower size. The species with big flowers (>3cm) is the narrow-leaved Campanula persicifolia (Peach-leaved Bellflower, left) commonly self-sown in villages and towns. Smaller flowers, with stigmas less than half as long as the style, is something else.
The last two in this group are dainty plants separated on their roots and basal leaves. Campanula rapunculus (Rampion Bellflower, left) has a thick tap-root, narrow inflorescence and petiolate basal leaves. C. patula (Spreading Bellflower, right) has an open panicle & thin roots
Now for the species with pores near the base of the capsule.
The first species in this group is easy and really distinctive. This is the chalk down-land native Campanula glomerata (Clustered Bellflower, left) which has sessile flowers, lacking a distinct pedicel. Plants with pedicels are something else.
Next, measure carefully the width of the calyx teeth. Are they very narrow, linear to filiform (<1mm wide at the base (left)? Or are they wider, ovate, lanceolate or triangular, more than 1mm wide at the base (right)
There are 3 species with filiform calyx teeth. The familiar acid-grassland native Campanula rotundifolia has round basal leaves (hence the Latin name) but the stem leaves are linear and untoothed (left). Toothed, broader stem leaves (right) are something else.
The other 2 species are easily distinguished by the way they hold their flower buds. Campanula rhomboidalis (Broad-leaved Harebell, left) holds its flower buds erect. C. cochleariifolia (Fairy’s-thimble, right) has its flower buds hanging down (pendent).
Now for the species with rather broader (i.e. not filiform) calyx teeth. The first decision is whether your plant is short or tall (>50cm), decumbent (left) or erect (right). This is usually as easy as it sounds.
First, the 4 decumbent species (stems prostrate with the tip turned upwards). If the diameter at the apex is much less than the length of the flower, you have the common and abundant dark (Oxford) blue Adria Bellflower (“port” = C. portenschlagiana). Otherwise something else.
There are 3 species with bell-shaped flowers where the diameter at the apex is equal to or greater than the length. The small-flowered plant (<20mm across, calyx teeth <6mm) is Campanula garganica (Adriatic Bellflower, left). Larger flowers are something else (right).
Finally amongst the decumbent species is the commonest of all: the plant with pale (Cambridge) blue flowers, shaped like stars, so common on walls in town: Trailing Bellflower (“posh” = C. poscharskyana). And see below.
The other decumbent species is Campanula fragilis (Italian Bellflower, right), has much wider petals (10mm at base, rather than 5mm)
The last group are the tall (>50cm) erect species. There are 4 of them. The only hairless one is Campanula pyramidalis (Chimney Bellflower, pictured). It has erect (not pendant) capsules, and a dense pyramidal or cylindrical inflorescence. Hairy plants are something else.
The most famous (infamous in fact) of the tall hairy species is Campanula rapunculoides (Creeping Bellflower). Christopher Lloyd calls it ‘the ultimate Trojan horse’. Graham Stuart Thomas says it is ‘the most prolific of ineradicable weeds’. Calyx teeth spreading or reflexed.
The last 2 tall species don’t have rhizomes and are much less invasive. A hispid plant with sharply angled stems and cordate leaves is Campanula trachelium (Nettle-leaved Bellflower, L). A softly hairy or glabrous plant with cuneate leaves is C. latifolia (Giant Bellflower, R)
So that's Campanula. There are 4 tall species and 4 decumbent ones in the group that has slightly wider calyx lobes, and 3 with filiform calyx teeth. There are 4 species with apical capsule pores (including Peach-leaved, below), and 2 with complicated, 10-lobed calyces.
