front/back wheels?Or all the same? Basically, i reckon my car is driving funny - a bit loose or baggy if you know what i mean? It's a fairly fast sporty car and is a 2002 plate with just over 60k miles. Is it a question of grip or a need for new tyres? Or do the front vary from the back? It makes me feel uneasy, like when you get stuck in the divots/tracks caused by lorries in the inside lane on the motorway. Any technical advice would be greatly appreciated.

The tyres should be the same diameter - but the thickness can vary !

The tyres all round on your car should be 225/45 WR17. Check that the correct sized tyres are fitted, and they're at the correct pressure (2.2 bar front, 2.7 bar rear)
Try the OE fitment Dunlop SP Sport 2000E tyres - the 'tramlining' that you're experiencing can happen with some tyres; I particularly find that Pirelli tyres are generally the worst for this, I'm also pretty unimpressed with their overall performance.

...get your bottom arms checked...[it may pull one way when accelerating then pull the other way decelerating]you may have a worn ball joint/suspension bush....nothing to do with the wheels...

Some BMWs have bigger tyres on the back to the front and it makes them really hard to drive in slippery conditions like icy roads, especially if they are rear wheel drive like some are.
If it feels a bit funny on the steering it might be worth while having the tracking or even the wheel balance checked. If you go to a decent tyre dealers (like not Kwik Fit or ATS) they should be able to spot if you've got a problem with your steering rack or anything like that. They can check the tyres too, they may be under inflated, that would also cause that.