One of my front tyres is nearly slick in the middle (ok near the edge though) whilst the other is fine. I've only had the car a few months so guess it was run at too high a pressure by the previous owner.

Anyway, given current road conditions I think I will change it now, so does it matter if one tyre only is replaced, meaning one new and one slightly worn?

Also, how critical is it to get matching makes/patterns if they are the same rating?

If you mix and match tyres makes/models you'll get significantly less grip.

I'd want a matching pair front and a matching pair at the back as a minimum.

What car is it? FWD/RWD?

The most important is that they are rated the same, though it sounds like you already have that covered.
Probably also worth checking the suspension alignment to find the cause of the uneven wear pattern. Having said that, you're probably correct in thinking that it is nothing more sinister than too high a pressure.
In the long run, dooosuk is spot on about the loss of grip if mismatched. Definitely worth going with similar tyres on the same axle, though differences between front and back are not necessarily a bad thing: if you are looking to bias the mechanical grip then it is positively encouraged.

put the matching pair on the driven axle

on a 2wd, usually best to replace both tyres on an axle (so both fronts or both backs). day to day you wouldn't really notice the difference, but when you hit the brakes hard on that damp road one night, you'll be glad you replaced both.

always check tyre pressures when the car is bought, serviced or had it's tyres replaced… i've experienced far too much air in the tyres after all of the above!!!

Cheers, I'll a get one to match the (brand new) one in the boot and put the least worn one back as spare, even though they are more expensive than the current ones.

They are the driving wheels too (FWD) so better safe than sorry.