The common comfrey (Symphytum officinale) is a bit of a teaser! Although the usual colour of the flower is a creamy white it also occurs with pink or blue flowers, indeed sometimes a combination of the two. That is where the problem lies as the other frequently found comfrey, Russian comfrey has blue or pink and blue flowers! It is easy, therefore to confuse the two.
As is often the case with similar plants there are various small differences which the expert can spot straight away but for us casual observers this it is far more difficult. There is, however, a pretty good ready guide, although not totally conclusive, in that common comfrey is nearly always found in damp conditions, especially by still, fresh water. Being a garden escape one tends to find Russian comfrey near houses or along roadsides where garden rubbish has been thrown out.
There are a further eight comfrey species in my reference book but they are unlikely to be found in Dorset and many are, in fact, garden escapes and similar to Russian comfrey so, again, unless you are an expert you probably would not know it was one of these even if you saw it!Common Comfrey: a Russian agent in disguise