Full Tag watch service.

If you're going to use it for diving it'll really need pressure testing, and possibly other seals replacing.
Authorised dealer as said above.

My OH collects watches and has several Tags, we haven't touched their service department in years.

Find yourself an horologist, one that is registered and local. They will do a better job, not necessarily cheaper (they will use genuine parts bought from Tag) http://bhi.co.uk they will do much more than the Tag service centre (I've had watches come back from Tag in Manchester with issues, damage - bezel and hands also with faulty seals). They are very good at correcting/compensating but you don't want that, you want a good service.

Your horologist should give a similar if not longer warranty on work.

Our local one is about an hour away away and he's worth his weight in gold watches!

Any independent watchmaker should be able to help you with that. They use standard ETA Swiss movements (955 series at that age) which I can replace completely with an identical brand new unit for around £100. That gives an idea of how absurd their "servicing" charges are.

Also, do you have any reason to think it needs servicing except for "it's due"? Quartz watches aren't like mechanical ones in that respect (not that the makers will admit it) because they simply don't suffer wear.

Rather than being continually driven by the force of a relatively strong spring, they get moved once every second by a pulse from a very low powered motor.

Those pulses last around 1/1000th of a second and, for ALL of the rest of the time they're sitting doing nothing. They're only under load (and suffering wear) for around 1/1000th of the time they're working - the wheels in your 20 year old watch have only had about a week's worth of wear!

Even if it does suffer damage, as mentioned above, the complete movement can be replaced with a genuine replacement for less than one maker's service.

That means there's no benefit at all in having a quartz movement serviced before it shows problems except to the bank balance of the person doing the work.

The bottom line is, if it still keeps time and the batteries are lasting as long as you expect, get a battery change and a reseal / pressure test done because it doesn't need anything else. And I say that as a professional watchmaker who's talked a LOT of owners out of paying me for work that's not needed over the years!