1996 Vauxhall Vectra overheating

We bought this vechicle in February this year and it had already done 60723 miles. I had the cam belt replaced and since then we have driven it to Paris, Scotland, aroound Ireland and up and down the M1 several times between the midlands and London running perfectly.

We now live in London and yesterday I began to worry as the temperature dial began climbing to 60 and slowly to 100 and then the cooling fan came on.
In older cars you were able to fill the radiator to help prevent the engine overheatiing.

I checked for a radiator but found only inlet caps for oil, brakes, another that warned of high pressure something if opned, water for window wiper and of course the dip stick.

Can you please give advise of possible cause for engine overheating, and what can be done about it.