As this concerns property damage, the normal escalation route for complaints about VM (to CISAS) isn't an available option, as property damage is outside CISAS' remit. So if you've got a big bill, and it seems VM should pay as its down to their faulty workmanship, then you need to consider one of the following:
1) Contact Citizens Advice and see what your rights are. Free, sometimes excellent, sometimes not quite so useful
2) See a lawyer. About £100 for the first consultation, but potentially then leading down an increasingly legal route with high expenses. If you end up in court and win you should get the expenses back, but this can be slow and stressful. In my view lawyers don't have a good reputation for handling small consumer rights cases.
3) Join Which Legal. £99, but that's for a year's unlimited legal advice. They'll explain your rights, what wording to use in letters, and if you do need to initiate legal action they'll help you with making a claim yourself. You could try adding that to the cost of the claim against VM.
In terms of the residual damage after cleaning, if a trade cleaning service has not removed it, I suspect you're stuck with it unless you're prepared to risk further more aggressive work that may cause physical damage to the stone. That may be unwise, since if that didn't work, I reckon it's unlikely a court (if it should go that far, which hopefully it won't) would agree to the ultimate fix of relaying new stone. I think you might reasonably demand compensation for (1) poor handling of the original work, poor handling of the complaint, the cost of cleaning, and an amount in respect of the residual and probably permanent damage, but all of this is getting complicated and you now need professional advice. Personally I'd go with Which Legal as they helped me secure a £30,000 settlement in an employment dispute (the best £99 I've ever spent), you might want to see if Citizens Advice can help first. And perhaps give the forum staff this last chance to get you a settlement offer before taking matters to the next level? VM have a history of paltry compensation offers - if that applies here it's your choice whether to take a low offer because that's the path of least resistance, or to hold out for what you think is reasonable.