Hi Robert, You can buy all new bit's from the likes of ebay including the heat exchanger etc and make one up as you say for well under half the price. However what put me off using webastow type heaters they are very expensive to get serviced when they go wrong and not really intended or suitable for hot water on demand wfp work due to the nature of how the heater works. That is why many just use the return to tank option instead and heat the water in the tank rather than trying to use it on demand.
Yes I know many have been sold and ionics were the first to start using in their van mounts but many wfp users over the years have realised their pitfalls too.
I'm sure my mate lance has still got a broken pf one in his van that he wants to get rid if interested
I might be interested if its cheap and depending on whats broken .
I did try a gas heater L5 that poleking sent me for free (top bloke) , but it was erratic and now doesnt fire up at all , and im not super keen on the gas bottle in the van ...... i did like working with hot though .
it would actually be cheaper for me to get a new tank with an immersion boss in it than it would be to buy a new diesel heater !