This is direct from the London Heathrow Aerodrome Book and shows the regulations regarding APU usage.
3.4.3 Auxiliary Power Units (APU)
procedures
3.4.3.1 APU must be shut down at the earliest
opportunity on arrival on stand.
3.4.3.2 APU must not be used as a substitute
where FEGP is adequately provided
and serviceable.
3.4.3.3 3. APUs are not permitted
to be used between 2330 and 0600
(local) on:
Cargo Area Stands 601-609
and 611-616;
Stands 401-403, 429-432
and 463, except in an emergency.
3.4.3.4 No APU is to be left running
unless either a qualifi ed person
is in attendance or the APU has
both an auto-shut down and an
auto-extinguishing facility.
It is fairly common to find APU restrictions, engineers may have different restrictions, but for crews on everyday operations it can be quite restrictive.