I checked my tyre pressure before setting off on a long journey and the rear left tyre was showing 24 when it should be 32-33

I checked it again after a couple of weeks and it was showing 24 again

Slow puncture and if so would it be advisable to take it to a Lexus dealer or just to any tyre place ?? (Kwik-fit, ATS )

You can take it off yourself and have a look if there's a nail in it somewhere - depending on where it is you then know whether it can be repaired or if you just need to order a new tyre

And yeah no point going to the dealer for a new tyre, they'll rip you off, just go to a decent tyre place.

You never know about the dealer. Sometimes they do come through surprisingly well in times such as these. As for puncture fix (hopefully it will be a leaking valve) - make sure the place you use is properly BS certified and will warrant the repair.

IIRC the best method is to push a solid rubber bung from inside out to fill the hole.

Take it to a place that you trust - so you don't get ripped off. Failing that, no harm in getting an opinion at Dealer and then going to another Fast Fit type place to see what they suggest. if you have Costco Membership, then they'd be the best (and cheapest) place to go to replace a valve; altho' they don't puncture repairs.

If you are around Milton Keynes there is a place I could recommend.

take it to a garage you can trust, if it can be repaired you don't want to be sold a new tyre

Wise words indeed.