Just a thought-is there any difference (in ease of laying) between 40mm or 60mm floor tiles as both sizes are available in my chosen colour,thanks in advance for any advice

A few pointers.

In a big room use big tiles as they look better and any dodgy uneven ones will not be noticed.

Large tiles mean more cutting and more waste so for smaller bathrooms use 30x30 31x31.

Use a plain tile and you could salvage your offcuts with a pattern such as 4 tiles making a circle you need to plan it so its equally centred and that often means starting in the middle somewhere.

I would for example work it out like this.

Your room is 4500 wide. So thats 14 rows plus a cut for a 300mm tile or 11 rows at 400 with a narrow cut. Assume your joint is 5mm max.

If you have an odd number of rows you can equal the gap at each side of middle.

I would do the 11rows.

If you are diy ing just be careful to get a tile cutter that does 600 tiles then you are covered to vary yout tile sizes around the house.

Believe me using an angle grinder to cut a 600 tile is donkey work and needs patience and skill because its bloody dangerous.