Thanks guys - that was the one - 47 quid plus vat iirc... I asked the mechanics at the commercial dealer (much better than the car side... ) if anyone had fitted one of these - there was much sucking of teeth and 'ooohs' and 'aarrr's' - followed by one guy piping up that his mate had one that needed replacing, and the mechanic had been putting him off for months as it was "a pig of a job"
remove upper and lower dash panels (about 8 screws, all easy to get to)
remove 6 screws from heater matrix box (inside cab)
open bonnet and remove two pipes (bit awkward as the windscreen wiper motor is in the way)
yank heater box out of dash.
remove unit and replace.
shove it back in, connect pipes and repalce dash... total time - 1.5 hours :bannana: heat and cold restored!