subsp. microcarpum on the Ripon Canal. Image: M. Wilcox
There are four subspecies in the UK & Ireland, all of which are detailed in our standard floras. So they are doable, but only with mature fruit (so now onwards is a great time to collect fruit) and being aware of the pitfalls in the key. The split between subsp. microcarpum and subsp. neglectum is particularly tricky as it relies on a judgement as to whether the individual fruit are 'shouldered' or 'gradually narrowed'. See Mike's photo of the former and note the difference between the mature fruit and the immature fruit. Subsp. microcarpum might perhaps be better described as having a domed head as it is not markedly shouldered, and subsp. neglectum as tapering. There is a great picture of the latter (and subsp. erectum) here.
If you would like to get in touch with Mike his address is in the BSBI members handbook (Members area of the website) or drop him an email. Fruiting heads can be put into a paper packet or envelope, and full details of the location should be provided (there is no value in a record if we don't know where it comes from!). You can email Mike at michaelpw22[at]hotmail.com (replace the [at] with an @).