I've just stumbled across the fact that the diploid populations of Tolmeia menziesii have been segregated (ten years ago) as Tolmeia diplomenziesii. That that raises the issue of which species is present in the UK (or are both).
Stace (2010) lists Tolmiea menziesii as having 2n=28 which would be correct for the tetraploid T. menziesii rather than T. diplomenziesii (2n=14). Of course whether the data was observed from British specimens I don't know.
According to the online Flora of North America (http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=133093) there are some other differences which might be easier for field botanists to work with.
This might allow redetermination - set up a Tolmeia project and run through the records? There prob won't be many of them...