Hi Mike it could be down to corrosion around where the tyre bead seals to the rim. You have to remember the tyre flexes as it rotates - more often at speed obviously.
As a matter of interest are the wheels alloys
Head under bonnet all weekend trying to get the
fault nailed down...
No the wheels are not alloys just the standard type.
Its very annoying to not see any leaks while the wheel is submerged in water though.
Is it possible to have the pressure sensor in the wheel moved over to the spare?