More heater problems!!!!

Well... only a few days after saying in the new members section how good my 97 300tdi was ...
ive got problems!
Lent it to a friend for the day, & while he had it the water pump started leaking.As a temporary fix he used Celit, a radweld type product.
Since then the heater has started blowing really cold, whereas before it was lovely & warm.
The gauge reads normal, doesnt seem to overheat at all, just the heater blows cold.
Tried bleeding using the brass bleed screws & it does seem to get a little warmer, but then goes really cold again.
Its allmost as if its sucking in air from somewhere.
Read a few posts on here & im aware this seems to be an early sign of head gasket problems, but was fine before the pump started leaking so im kinda thinking maybe the water pumps not circulating the water correctly.
Im gonna have another look on boxing day but was wondering if anyones experienced anything similar.
Any suggestions at all would be appreciated
Steve

Has the sealant blocked your matrix somehow. Maybe a good flush out will improve things.

Thats what i thought at first, but when you bleed the system it seems to get a little warmer before going cold again...

have you fit new waterpump,and wot sort of friend chucks crap like that in your rad anyway