For all you turbo charged nutters out there
I have found some garages that still sell leaded petrol
This stuff has an octane rating of 102(ish) RON - optimax is 98 and may allow you to advance the timing a degree or 2 for a few more Geegees
However, its not gonna be cheap - expect to pay more than optimax for it, and its not local either. Nearest places are Wrexham and Hawarden
to download to list of UK sellers in .pdf format.
May take a trip up to the Hawarden one at some point, and fill a few drums