Indeed - and, as I said, even if polarity is correct, the regs seem to take the view that DP isolation is needed for TT - presumably on the basis that, in the absence of (satisfactorily functioning) RCDs to clear the fault, a CPC etc. could, due to a fault in the installation, be at a potential very much above the potential of neutral (or, put the more relevant way around, the neutral could be at a very different potential from the CPC etc.Depends how worried you are about the possibility of reversed polarity. Depending on your opinion of the average standard of installation and testing of central heating installs, you may well be concerned about neutral being 240v from the boiler casing
Kind Regards, John