Seek better mould cleaner than "disodium octaborate" ?

Kill it with straight chlorine laundry bleach (gloves, goggles, respirator) and an old toothbrush. Rinse and dry. Apply "Polycell

If you had black grout this would't be a problem.

Don't be silly, it's clearly obvious that you'd then get white mould :)

1. That's probably as strong as you can buy.
2. We don't have obscure wholesale chemical warehouses in the UK - not that any general public can get to anyway!
3. Try good old bleach, or a solution of Bio-tex might work, or Vanish
4. Best still, rake out the grout, clean the tiles and use a good mould resistant one grout in place : go to plumbers merchants - don't use ready made, it holds too much water - encourages mould, use one that has to be mixed by weight (cement based) - and use scales! Mix about 300g at a time (about 55g water IIRC) as it sets in 20 mins! My bathroom gets well damp, not a trace of any mould, even with the NW weather (raining again...)

Do you know what ingredient in the Bio-Tex and Vanish makes them particularly suitable?

Not a bleach, surely?

Yellow pages. Take telephone, credit card, and intone "I'd like to place a small private order for XXX by credit card" till you get a positive response.

Bio-tex is an enzyme formulation. Vanish (the powder form), will probably be a percarbonate (i.e. mild non chlorine bleach)

LOL ;-) This is the UK! - HSAWA, COSSH, IPPC... = no private sales. I happen to work for Alfa Aesar / Avocado / Lancaster. We do get some calls like this (one or two a year), our friends with the blue flashing lights are usually quite interested...

Maybe things have changed. I quite easily got a barrel of sulphuric acid, and H2O2, 5 years ago or so.

You may well be able to now.... Depends on what the use is. To buy chemicals as "chemicals" is nigh on impossible - there are only (now) 3 main suppliers for small to medium amounts - us, SAF, and Acros. One *might* be able to buy "battery acid" - but that would be ca. 25% w/w sulphuric, peroxide - yes as dilute solution for hair bleach, and a small selection of "photographic chemicals" can still be got from some helpful camera shops, plus a few other odds and ends from (helpful) independent pharmacists. Anything else will attract the attention of the HSE and the EA (especially the EA, who will want to know what your waste disposal plan is - and flushing anything that is not "domestic" down the toilet will not endear you to them....)