Geranium molle has small, bright pink or almost whitish flowers with ten anthers. The leaves are dissected less than half-way and it has both silky long and short hairs on the petiole and peduncle. It's common almost everywhere.
Geranium pusillum has rather duller, more mauvish-pink flowers, with only five anthers and the hairs on the stems are all short. The leaves are somewhat more dissected than G. molle, but not as deeply cut as G.dissectum. It particularly seems to like areas which are summer droughted and is locally frequent especially in brownfield sites. It is often under-recorded, being mistaken for G. molle.
G.dissectum usually has small, bright pink flowers (the photograph above is of an unusually pale form) and has deeply-dissected leaves. It usually has a mix of short and long eglandular hairs. It is surprisingly variable and can be difficult to identify vegetatively, but is frequent to abundant in a variety of disturbed and grassy places, and is often a weed of arable fields
Geranium columbinum is probably the rarest locally, and is superficially similar to G.dissectum, but it has even more dissected leaves and short stiff eglandular hairs. Most pedicels are more than 2.5cm long, giving it the English name of long-stalked crane's-bill. It occurs regularly at Bedford Purlieus and has also been recorded recently from Castor. It normally grows on limestone soils.
Geranium rotundifolium seems to be turning up all over the place at the moment. A large patch has recently appeared on a route that I've walked regularly for about fifteen years. The petals are rounded at the apex. it has glandular hairs and the leaves often have red spots in the sinuses and at the ends of the lobes. It normally grows in bare stony places that are parched in summer.