After a few posts on the Elise forum it seems like its water getting on the plugs and/or distributor. A liberal squirt of WD40 and she seems a lot better but still not perfect. She's off to see a local mechanic tomorrow morning for a full check. Might be a plug seal or something leaking.
Also found out where the waters coming in inside the cabin - the top of the A pillar, where the roof meets the screen. Will remove/refit the roof as soon as I get chance. Reduced to a teatowel to mop up at the moment.
Not happy at the moment. Gonna cost more Â£Â£Â£ tomorrow when she sees the mechanic