Theres a seller on Ebay selling brand new Propex heaters - these run on propane/butane (from your cooking gas bottle) and can be mounted under the floor and the hot air ducted to wherever.
The price is good for a brand new one (£594) with 12 month warranty - Eberspacher & Webasto Diesel/petrol powered ones go for up to £2-400 for second hand ones with no warranty and £1200+ for new ones

It is a lot of money, but the convenience of being able to switch on what is effectively a fan heater whenever you want, and even run it all night if needed, does cost



Note that Steve (nearest) failed to get his Eberspacher running due to a low battery, whereas mine (behind) was nice and cosy - cosy enough for Steve to come knocking on my door in the morning to come in for a warm

Anyway, if anyone is interested, the link is HERE
Note: I have no connection with the seller!
Cheers,
Robin.