MX5Ireland

I've tried some of the cheap liquid metal you see in shop counters (can't remember what you call it, sureweld?) which was pretty useless, then I tried some more expensive stuff I got from Amazon, it seemed to work well, it adhered and didn't leak for days, but eventually the fuel penetrated it and the adhesive was gone entirely and it peeled off easily. I really want to avoid swapping out the fuel tank, has anyone any experience with anything else? AFAIK welding is out of the question RE safety

You will have to swop the tank or safety reasons.The exhaust runs under the tank so any leak could cause fire and you dont want that!
We have tanks if you need one. Its not huge job to swop the tank but you would need car on ramp/lift really