For anyone not able to pour an untested snake oil tank sealer into a leaky tank themselves, this guy is able to do the work for you: http://www.tankcareproducts.co.uk/

Seems surprising that anyone would be willing to pay 50% of the cost of a replacement tank on a so called repair, carried out with a product which doesnt seem to have been properly tested in any way?

Its worth noting that using any flavour of unmodified epoxy resin inside a GRP tank will most certainly result in failure, unless applied very heavily so film thickness is around 3-4mm, which should be enough to resist cracking.

Finally to ensure a good secondary bond when sealing GRP tank interiors, its crucially important to mechanically abrade the whole of the surface.............scouring with acetone will not achieve this, and will also not remove many decades of oxidised fuel residues, to which sealer's will not adhere.