I've got a mini cooper & they dont have the mini run flat tyres on it, they have 18inch tyres on them. I've had the car for about a year & it's always had these tyres. But recently (past few months) the tyre pressures just keep going down really really quickly. Every week I'm having to pump them up & usually they're on as low PSI as 5! Last week I got 'eggs' in 1 of my front tyres & thus had it replaced. But even this new tyre is going down within a week & you can visibily see that its quite flat!

Does anyone have any ideas as to what this could be!? I did originally think I just needed all new tyres, although the fact that this new tyre is still going down quickly seems to suggest otherwise ?!

If your alloys are corroded enough, that can cause loss of pressure - I have had that before, but the appearance of the alloys will be pretty bad by the time this becomes a problem. The corrosion prevents the tyre from making an airtight seal to the rim.

If you've got the lacquer/paint on the alloys lifting/flaking off at the rim, then this could be the problem. It would explain how come a new tyre has the same problem, but I would have expected to tyre fitter to notice and mention the corrosion, if it was bad enough to do this.