Yes, I agree with Jules. It must be the coil starting to fail. If you ignore it, it will only get worse. Try fitting a Lucas DLB100 coil (available from the Green Spark Plug company or from Autojumbles like Beaulieu) and this will cure the problem. I have had the problem before and a new, not an NOS one, is the only cure. Last time I had a coil fail was this year when I went for an MoT and the WOT2 died on me about half a mile from home.
On my WOA2, I had the radiator recored when I was restoring it. This still pushes out 2 pints after I have filled it up but doesn't overheat and runs well. It's just a V8 characteristic, don't worry about it. Just keep an eye on the temp gauge during summer months and keep the speed down.