There is a lot of cow poo on the roads in that region. Are you certain the locals have not left a deposit on your cat's which is being burnt off ?
I had that in the area around RAF Coningsby where I got a load of cwap on the exhaust/pipes. The smell was quite strong but made worse on full power moments as the pipes got hotter.
I ended up putting it up on axle stands and jet blasting (very very carefully!) down the pipes.
Now this is an avenue I did not think about! Quite possible and would explain why it was worse on full throttle.
I shall have to get the hose out when I am back, to be honest it needs it with a week of bug splatter and rain!