Octavia VRS heating blowing hot then cold

wondering if anyone can help please. bought a 62 plate VRS TDi with 130k on and a fantasic service history on saturday. during test drive didnt notice anything a miss

on sunday i noticed it has a strange problem with the heating. from cold as the car is driven, and the temp raises on the dash, it blows nice hot air out. as the temp gauge goes past 80-85 degress the blowers start blowing cold air. this happened today as i sat in traffic, the temp rose to 90 - no higher - and the blowers go cold. start driving again and the temp dips below 80 and the blowers go warm again.

once sat on the drive, and the temp on 90 degrees, the car is blowing cold air. turn the heater off for 5 mins and then turn back on and hot air comes out for 30 secs and then goes cold again.

the fans at the front are working when the air con is on an doff when the unit is off. the radiator is cold even with the car running on the dirve. im in hull, uk and the outside temp is about 8 degrees.

i did the flap reset and heard them all moving but still blowing cold. car was serviced 2 weeks ago and all levels are perfect. no mayo in the oild etc. radiator reservior does pressurise slightly but doesnt boil over or lose water.

car is currently in the garage. been in touch with the (private) seller and he didnt have any issues and is communicating with me. i'll update when i know what the garage says but am i missing a simple trick?

thinking worse case head gasket, car needs a belt and pump to be fair so not the end of the world. hopefully seller mite through me a bone too. best case a temp sensor?