I think the 2 symptoms are probably unrelated. I would have expected to see some evidence in the oil if the head gasket had gone and you would notice a deterioration in the running and maybe more smoke in the exhaust.
My guess would be that the air was not bled out when the radiator was refilled and it can take couple of runs to find it's way out on it's own if you don't bleed it.
This has happened to me a couple of times on my Silvertop Zetec when I've changed the radiator and pipe work but it did eventually find it's own level and settled down.
Keep an eye on it and obviously investigate further if it doesn't settle down.
How much water do you need to add to top it up?
The low engine temperature is interesting because I had the same problem in the early days.
When I picked the car up from a dealer near to you I noticed that it took a very long time to warm up and indeed in March last year the temperature never really got above about 80 degrees on a very cold day.
I was told that some cars are like that and we discussed whether the thermostat should be checked but as the year wore on and the weather picked up the coolant temperature also started to rise up to about 85 ish (Which is still too low for a Zetec.) and my warranty period had run out so being new to Morgans I just left it.
Later in the year I started to notice some leaking from the radiator around the top hose so I decided to fit a new Sifab unit and at the same time I changed some hoses and decided to take a look at the thermostat.
I dropped it in a pan of boiling water and it started to open then I let it cool and it closed so thermostat working ok.
However someone had drilled 2 holes in the flange and this had the effect of allowing water to bypass the thermostat and in effect leave the thermostat open at all times.
In hot weather this might have the effect of keeping the water a bit cooler but in cold weather it stops the coolant getting up to the proper working temperature which is very bad for all engines but especially the Ford engines that are designed to run around 92 - 95 degrees or even slightly higher.
I was told this is a mod that some people do but for the life of me, I cannot understand why.
I changed the thermostat without holes and the system now warms up quickly to the correct temperature and stays there regardless of the ambient temperature. That is what Ford designed it to do and if the rest of the cooling system is in good shape that is the way it should be run.
You may not have the same strange mod that I had but the symptoms are the same and whilst the thermostat is a bit fiddly to get at, it should be checked.
I don't understand why your dealer would not check the thermostat with these symptoms especially as your system was already drained but my dealer was also reluctant to strip it out to check and I found the problem myself.
My Zetec is a Silvertop and your Blacktop may not be exactly the same but I think the thermostat operation and plumbing is very similar.
Good Luck
