THTC is Total Control heating. (never seen it referred to as that before, had to google to be sure)
Total control is a preserved tariff no longer available to new customers. It was popular in the 1980's when they were trying to improve the image of storage heaters and pretend a house with electric heating was as good as central heating.
Anyway the important bit is there are three metered tariffs as part of this package (and often 3 consumer units for the seperate bits)
Firstly there's circuits metered at the standard rate. This is all normal sockets,. lights etc. In this case the standard tariff is a bit higher than it would be if you just had a normal single rate supply.
Secondly there are off peak circuits. These are only energised at night, and are for things like storage heaters and immersion heaters. You also get a 2 hour boost to these in the middle of the day. This is charged at a much lower rate.
Then there are the "total control" circuits. Now this is the special bit that makes total control an attractive tariff to hang on to if you have it. All the total control circuits are metered at the cheap rate, but are available 24/7. This is supposed to be used for heating appliances like panel heaters, feature fires, and you can also connect electric showers to this.
So, the availability of cheap rate electiricy 24/7 for heating makes total control worth keeping.
Now, have a look round your flat. My guess (based on what I normally see) is you will have storage heaters in the living room and kitchen (big things that stand on the floor) and panel heaters in the bedrooms (much slimmer and hung on the wall)
You can be sure that all the heating you use, whatever time of day will be metered at the cheap rate.
So use the fixed heaters you have. Whatever you do, don't plug in free standing plug in heaters, they will be metered at the peak rate and will bbe much more expensive to use.
And as with any heating system, just using the heaters when you want them will be the cheapest option. That's why they normally fit panel heaters in the bedrooms rather than a storage heater as you don't want the bedrooms warm all day long.