my misses Vauxhall Astra 2000 reg has no heat from the heater

there is a known head gasket issue, which we have put some steel seal in, and now the water level does not go down in expansion bottle

when I put the steel seal and ran for 30 mins, I pretty sure I did not see the temperature needle move that much, if at all

our local garage just keeps blaming it all on the head gasket

in other cars I have had it has been the mechanical stat stuck ?

misses has also said she can see the oil warning lamp on dashboard at times, when idling ?

all advice appreciated