Run out to warm the oil before changing it and after about 7 miles it developed a massive misfire - a bit like a very aggressive rev limiter. Made no difference whether the engine was under load or not, just wouldn't rev beyond about 3k. Behaved as normal below 3 k.
It's on twin 40s, Aldon no vacuum distributor and Lucas sports coil.
My money's on the coil.
Any other suggestions?

I'd start at the ignition, identify which cylinder then start looking for breaks, wet, dirt, cracks anywhere. Check leads by substitution, is the dizzy electronic?

3,000 rpm is where the main jets take over --- clean them and check they are tight

Ignition misfires-show up at 1500-2500 at full throttle as this when the spark voltage demand is highest.

Check everything is tight inside the distributor and there no connections that are iffy under vibration.

This thread might help with checking plug leads http://www.locostbuilders.co.uk/viewthread.php?tid=204630

There are more test that can be done on ignition high tension system but need an o- scope or KV meter.

I lost a grub screw on one of my 45s that caused something like that. The screw held one of the Aux venturies in position. You'll easily spot if one has moved, have a look down the chokes and just check if one of the support "bars" has twisted out of position.

Blocked fuel filter? Mine did very similar when my filter internally collapsed.