Full Service

Inspect & grease hubs, headset, bottom bracket and pedal
bearings (where appropriate and not cartridge units),
possibly replacing any balls that look dull.

Remove, grease, refit: pedals, seatpost, quill stem.

General checking, adjusting and lubrication of brakes,
gears, cables, wheels and tyres. They might routinely change
cables and brake blocks (even when this wouldn't be strictly
necessary). Check chain for wear, possibly replacing it,
possibly sprockets too if well worn. Nothing more advanced
unless something was broken.

~PB

Richard Bates said:

If I take a bike to a LBS for a "Full Service", what can I
expect them to do?

Sorry, it may be a long answer. I'm just wondering if
anything will be done, which I am currently unaware should
be done, that I cannot (as a reasonably decent bike
mechanic) do myself (other than true wheels and do whatever
needs doing to a headset).

TIA

I have a feeling that I'm about to discover that I'm
actually a very poor mechanic when I see a list of jobs
that I would never have thought should be part of
maintenance!

I doubt there'll be much that you couldn't have done. I like
to have my bike professionally serviced every now and then
for two reasons.

Firstly, it means the gears will work for a while. As far
as I'm concerned, derailleur gears work by Black Magic
and can only be made functional with cantations learned
by LBS wizards.

Secondly, it's simply reassuring to know that a second pair
of eyes have had a look and might have spotted some
impending failure that I'd never have imagined. It's worth
20 or 30 quid once a year for that peace of mind alone.