water filler hosesize

OK, so I now have a length of 40mm hose,  a new filler and cap, please has any one got any good ideas for re-threading the new hose to the holding tank, I can only see a 3ft length of the old hose at the back of a cupboard where it attaches to the filler inlet, A/S did not make these things easy to refit, as far as I can tell the hose passes down by the propex heater  somewhere and comes out under the van, is it likely to be attached to somewhere I can't see or could it be pulled through (carefully)
mike
vwt4