A friend had some fuelproof 2 pack tank sealant and I put some over the leaking spot weld internally, let it cure a couple of days and the tank still leaked so realised it must be coming from some of the other dozen spotwelds.
Put a coat covering all the welds and now all is good.
Although the pump is only 3 years old a spare tank was not available which seems poor form - Bianco BIA-WP15A - but now that I found several of the welds had leaked it was probably a good thing they don't supply spares as a new one may have developed leaks down the track.
