Another one to look out for (albeit quite hard to notice) is misfires. If you have a VCDS scan tool you can check misfires. This can caused by something simple like spark plugs (£20 for a set), coil packs (£80 a set for good ones) up to injectors (mine set me back £800 a set!).
Also pop the bonnet and have a look at the insulation - if its soaked then you may want to replace it which will cost a bit. MK2 Leons have a common issue where it leaks so if you don't want a nasty saggy looking insulation you may need to replace it.
Other than that Brimfull and Legojon covered the other main bits.
Another thing to check is that the thermostat is working correctly. Engine temp should rise to the 90 degrees on the dial and stay there.
If the needle drops below 90 when driving but sits at 90 during idle it will need replacing.
Cheap part to buy but the labour involves alternator off and sometimes the intake manifold removing in order to change the thermostat.
(Just had this done on mine)
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Sorry to go on a bit of a tangent, do you have the part # for the thermostat? I might try do this whilst we're stuck at home.