D'oh! What is it with me and car fuel problems.
The MX5 has started to drip petrol from the filter. Looks like someone had broken the push on fitting on the end of the tank to filter fuel line. The clips are easy to break but also easy to repair. Rather than repair it they cut the end off and joined the line to the filter with a bit of rubber hose. This has now started to dissolve and wasn't a safe way to repair it anyway as it's a pressurised line.
Unfortunately to replace the line means removing the rear suspension and diff assembly to remove the tank first. Feels like I've been here before.
Decided to try to repair the line in the short term.
Made a brass double tail fitting that has to be pushed in while the pipe is hot (100 c) and a new end connector. This will be backed up with a short length of 8 mm braided fuel hose over the joint.
Hopefully this will sort it out.