"....said it is getting near the end of it's life"

If so, then the only part that would need relacing would be the
heater/fan unit. There'd be no point in ripping out the ducts. I think
Johnson & Starley are the only UK manufacturers of direct gas fired &
probably made your unit. Ring them, get numbers for a few local
installers and get costs for replacement and service for your model. I
think they may say it has lots of life left in it. Dump the Gas
contract if you find a reliable or cheaper alternative service
contractor.

Another option would be to fit a gas boiler with a water/air heater
battery in an air handler/fan. Fairly simple but probably beyond the
ability of most heating contractors.