[craggle]

Halfords do sell repair kits. I think they are a two part thing that you mix together and force into the holed area.
Metal needs to be dry though so tank may need to be drained. Not sure how well it would adhere to aluminium either.
If the SVA man spots it though he may be reluctant to continue as it is not a permanent repair. MOT people will probably feel the same way.

I fitted the steel tank as I was unsure how the aluminium welds would stand up to flexing over the years. Ssofar so good and it dosen't weigh much more as the steel is thinner than the aluminium. You just need to paint it before fitting it.

Craig.