The costs don't really matter, as NWH said the advantages are huge. He obviously does not feel happy with the gas and thinks it's dangerous and not legal. As he feels that way he is doing the right thing. Even the people who spend up to ten grand never regret it ( by the way tennent clean hasn't posted for a while has he).
Bear in mind with costs you don't have to have the hot turned on all the time, but supposing you did it the running costs would be £2 gas , £7 diesel to heat 600litre usage.
I'm in the £2 gang, but if I was ever stopped from using my on demand system i would go this route too.
Where do I find this adaptor thing?