Think I may have managed to answer my own question. I spoke at some length today with a manager at Sea-Chem (N.B. not Seachem!) a UK based firm specialising in waste treatments and bio-based cleaning. I can't post links but Googling the company name should get you there.
They have a product called Bio-Boost (which was mentioned earlier in this thread as a UK option) and after an interesting discussion would seem to be not just similar but, as its imported from the USA, identical to many other products discussed here, being multiple bacterial species and strains without any perfumes etc. on a 'bran' base media.
Each tablet weighs 8g and treats a septic tank up to a volume of 2,500 litres which is 660 US gallons. The guy I talked to stressed that a septic tank was constantly being wrecked in terms of bacterial colonisation by people adding various household cleaners and bleach. Therefore in an aquarium environment without such harsh chemicals being added daily, there was no need to dose anything like that powerfully.
As my aquariums are relatively quite small I'm going to be getting my digital scale, pestle and mortar out, and raid my SO's pantyhose stocks when my Bio-Boost arrives tomorrow. It cost £20 inc delivery which is still expensive by US standards, but then it should represent several years supply for me.
I'll post back with my experiences