Have issue with coolant went to check it this morning engine cold coolant level was just under minimum mark so as I screwed of cap the coolant tank filled right up to the brim. Cars a mk3 octavia vrs 18 plate.

The cap is screwing on with a bit of difficulty so just ordered another cap today hopefully that's the issue.. anybody else had same issue

Okay - be careful of this one.

Two things:

1) Don't fill up the coolant to the top of the tank. This will just cause overflow. Make sure the coolant header tank is kept to the MAXIMUM level and no higher.

2) Re: cap - they are all VERY stiff if you don't touch them for a while. Repeated screwing on or a tiny squirt of silicon lubricant on the screw part of the cap will be enough to free it off a little. A new cap is a good call, because they can fail. I've checked your profile and it doesn't say if your car is a TSI (petrol) or TDI (diesel). TDI's tend to use coolant when they do a re-gen. Its best to keep an eye on it on a weekly basis and check it at the same time you check your oil level.

3) Remove the silica teabag in the coolant header tank if you are able to do.

Keep an eye on the levels; if they drop off again then you've potentially got an issue with a faulty header tank or a potential issue with the header gasket.

As I read it, the OP did not fill it up to the top. The level rose from the minimum level to the top as he was unscrewing the cap.

Thanks @ords for being my checker. You are right. So - system is under pressure. A new cap is probably a good shout in that case.