I am posting this in case anyone else has problems and would like to share my experience.
I had to park my car up for 6 to 8 weeks as I was having a replacement hip fitted, December and part of January so cold weather.
First thing that I did wrong was to leave the battery connected, handbook says disconnect if not used for more than 4 weeks.
When I went to start the car the battery was flat, 8 hours on a trickle charger and the car started first time. When driving it I noticed that the heater fan was not working, very cold in the car and unable to keep the windows clear.
Visited my usual garage where all of the basics were checked, condition of battery, fuses, put it on diagnostic which showed very little. They suggested going to an auto-electrician as modern electrical systems are very complex and not to be played with, fair enough.
Booked in at auto electricians, in the meantime I rang Citroen and priced the parts, resistor £162 and fan motor £252. Then followed all of the comments from " those in the trade "
Nightmare of a job, dash to be taken out, airbags removed, one even said that the dash fixing bolts might be fitted from the outside and the windscreen would have to come out !!!!!!!!!!!!!
I was starting to panic as I love this car and really did not want to change it.
Never mind, the auto electrician deduced that it was the resistor and did a temporary fix, advising me that it would fail again so a new one would be needed, great service, got me back on the road. He showed me a picture of the resistor, got a genuine Hella OEM one from Ebay for £45. Happy days.
Now back to the motor, again I consulted Springfield Citroen in Gateshead ( in case it was the motor ) who quoted £421.74 to supply and fit a new motor, hang on, if the part is £252 that's not a lot of main dealer labour. About 2.5 hours they said, worst bit is getting the glovebox out, then it's pretty straightforward, a bit fiddly but not too bad. So no need to remove airbags or windscreen then ? No we've done a few, it,s not a nightmare.
Lesson learned, listen to those who know and have done the work before, sometimes the dealer is not only the best option but in this case very reasonable.
After all my C5 ( used as a taxi ) has only done 158k, lots of life left yet.
Hope this is useful.