Hello Peter K -
This has been a popular and mostly ill-informed topic on US car
forums for more than 10 years. I suggest an initial question should
be: Why was "lead" added to gasoline?
A good and concise answer can be found on
http://en.wikipedia.org/wiki/Gasoline under "Lead".
Current gasoline sold in the US matches the octane ratings of most
50s and 60s available gasoline, except the highest 95+
octanes. Ignition advance can be adjusted on most engines to avoid
detonation. I've never had any problems with my Imperial (Chrysler)
440 with a nominal 10.5/1 compression ratio.
The second feature of using lead was valve-seat "lubrication",
delaying seat recession, usually caused by high RPM use over
prolonged periods. This varies because of valve seat material and
design on any engine series/family. Many owners with
high-performance engines in the US specify hardened valve seats
during a rebuild, if that was not the original standard. Verified
reports of recession, however, are fairly rare.
Sorry I don't know much specific about Bristol engines, but the above
factors should apply to all gas/petrol engines.
Bob