Andy,
What you are going to need is something with Sodium Hydroxide in it, this is the stripper part of these sort of cleaner/strippers, I don't know the Isoflex, but not the Lithofin product is the right one for this either. If you have access to Sodium Hydroxide (caustic soda) I can tell you how to make a mix up, but would rather explain over the phone then put on here as if it goes wrong you can hurt yourself and also do some damage to what you are cleaning.
Let me know by 06:30 tomorrow morning if you need to speak on the phone, as I'm heading into London and need to get some work done before this heat kicks in again.
