A bit off topic but my fire suit has arrived which will be useful for any serious track or rally work the M3 may be used for in the longer term future:

As for the car, it looks like I’ve found a source of 5/8″ fuel hose from a place called Pirtek in Guildford. Thanks to Ian for recommending them, and for Ben Hughes for offering to go and fetch the part. I plan to try to get the M3 back on the road tomorrow.