p.s test the thermostat before you put it in, i had to do it all again coz had a cack one from GSF!
G
Second that! Stick it in hot water and see it open.
Another reason for hot air not blowing is that the heater matrix bypass valves may be blocked. These were installed as a recall to prevent the heater matrix going pow and unloading hot coolant over the passenger.
I'm not too sure how to unblock them tho, maybe a reverse flush or a new set?
it takes a long time sometimes maybe drive for 30 mins before temp gauge goes up to nearly halfway. then when i turn th fan on it blows warmish for bout 20 secs. coolant lvel is fine as car just been serviced
This sounds like your thermostat is stuck open or is lazy at opening. By running cool you are using more fuel too so it might be very well worth changing it as they are only about £4 and you need a new Seal too.