noticed that my rear wheel is not completely centred in the frame, about 9mm biased to the non-drive side.

everything is squared i.e. the frame aint bent!

I take it that it can be ‘trued’ to a more central position quite easily?

Just needs tensioning on drive side to pull it back.

Best take it to someone to do if you aint familiar with wheel building

10 min fix and it will feel a lot better when done as you will have lost a fair bit of tension no doubt

Fit the wheel back to front and see if the offset is on the same side, if so, it’s the frame, if it goes the same distance to the other side, it’s the wheel. Any thing else and it’s a combination of the two.