Clearing our attic for a conversion and looking at the cold water tank. Have emptied it and discovered the cold water and the toilet run from it. Any reason why I can’t just connect these to the mains supply as well? Should I even attempt this myself?

If there is a water cut you have no water at all stored in your house and the valves in the toilet might not like the extra pressure – but many houses do not have any tanks now and its simple enough to do

Your cold water tank probably feeds the boiler/hot water, so not as simple as just re-jigging some plumbing.

As TJ, some taps/shower/valves may not handle mains pressure water.

It won’t feed the boiler / hot water – that would be a separate tank

we have just had a combi boiler fitted, so don’t think the cold water tank is connected to that. The shower was still working after I shut off the supply from the tank so assuming it is connected to the mains.

IIRC the building regulations changed some years ago and it is no longer compulsory to have a cold water tank. It’s handy for flushing the loo in the event of a water cut though ( which is why it was a requirement).

My boiler is fed from the cold water tank. As far as I know thats normal for a traditional type boiler…big tank for hot and cold water, small tank for central heating header.

But the OP’s comment about having a combi-boiler fitted means that it probably is fed from the mains and not the tank.

Combi boiler is definitely sealed system – not fed from cwt.

Your ball valves in the loo cistern may not be high pressure valve so MAY need changed. You MAY have to change a ap or wo depending on whether they handle the higher pressure but the trouble is usually taps that aren’t designed to handle too low a pressure – you should be fine.