I know I am shamefully late in my feedback, I have a number of reasons but quite rightly that’s no excuse, please accept my apologies. Thank you to all who responded. A short overview of our fix… We could not find anybody to weld the crack & because the tank sits so low to the ground, it was not really safe to start trying out hot knives etc… ourselves, luck was definitely not on our side around that time so we didn’t want to take any chances. We ended up doing a “temporary” fix ourselves. For information the crack was on the outside, right in the corner & the tank was not accessible to seal from the inside also to note we do not drink from the tank, only washing dishes & showering etc..
We started by drilling a small hole either side of the crack to stop it propagating then lightly sanding around the crack to get a key & thoroughly cleaning the area with acetone. We then used JB plastic weld to seal. When that as fully dry, we covered a large area over the crack with Soudal Fix All Turbo bonding adhesive. Thankfully this held to enable us to continue with our journey, we ensured that we never filled the tank more than half way & we supplemented our water reserves by purchasing a large jerrry can. To our amazement the seal held but we will look to replacing the tank as soon as we can. Hopefully this information may help someone else. Apologies once again & kind regards to all