Have you checked you oil recently to see if the level has gone up?  Is the
oil rainbow coloured?.  If so the cylinder head gasket may have gone and the
water is now mixing with the oil.  Unless you've got the tools, a lot of
time and a Haynes manual you will need to get this looked at professionally.
A cylinder head gasket kit isn't expensive, but the labour getting the whole
thing fitted is.

If the oil seems fine, check your water hoses.  Garages sometimes disconnect
them in order to empty the old coolant before replacing with new.  They do
this with the engine running in order to pump out the old coolant and quite
often don't tighten the hose straps up fully.

When you use the heater you open up a valve allowing the water to circulate
through the heater matrix and piping.   It can be the case that when you
fill up the radiator, the heater valve is closed, hence you are not filling
the entire system.  This can also cause air to get in and the effect of
colde then hot, then cold, then hot, heating!

Check underneath the car after it has been stationary for any leaks.  This
is the most obvious thing to check.

Hope this helps,

Al.

And Elvis was sitting next to Kaz in the spaceship, which I thought was
kinda weird, but then they turned to me and said:

Look closely at the heater water valve, where the operating rod passes
through the seal. These often develop a tiny leak past the seal, which
stops when the heater is set to cold.

If you fill it up to the top it will go down a bit, when it warms up the
preasure pushes some coolant out, the level should be about 20mm below the
top of the filler, posible places to check for leaks,  hoses, the rad
itself, bypas hose, ( between waterpump & cyl head,) heater radiator, cyl
head, core plugs, heater valve, water punp, never use rad weld, its a botch,
not a fix, also check the rad cap,

Steve.