size of a 5p is small but for a hole in a tank is conciderd large most tank leaks or holes are pin pricks and easy spot/mig welded in fact small pin hols can be sealed with a rubber washer and screw ..
the size of a 5p may be better to actualy mig weld a small peice of metal plate over it ? infact use a 5p coin and mig weld round the edge of that .
if I was you find a good or ok welder or garage clean up the area of the hole drain out and dry out any fuel and vapor and just wonder in with the tank in a bag and ask how much to weld seal this hole ..
chances are they will say here ill do it now take 5 mins give us a £10 er and a realy awsome bloke would smile laff a little and say here ill do it for free 2 min job
