AstraVanMan wrote:
I live in a block in S London, in pretty much exactly the same situation
as the OP. I am also a director of the management company, and we have
discussed a barrier or remote gates etc.
When we looked at automatic gates, we had to also consider access for
legitimate visitors such as the milkman, postman, delivery drivers etc, as
well as doctors and emergency vehicles, and legitimate residents'
visitors.

If there is sufficient space to separate incoming and outgoing traffic
(ie not a single-track driveway) the emergency vehicle access can often
be sorted by having sprung flaps in the incoming lane, which can be
driven over by vehicles entering the property, but cannot be driven over
easily in the 'wrong' direction, and having the exit controlled by barrier.

If the non-resident parking is caused by workers in nearby businesses
(as it often is) they will tend to arrive and depart at regular times. A
few days of someone standing at the entrance in a high-vis jacket
(either a resident who feels up to it and has the time, or a hired
'security' person - which does not cost much if you get someone from the
jobcentre or nearest gym, you're not trusting them with keys or inside
access or anything) and the unauthorised parkers will be deterred from
parking in the private car park and will get into the habit of parking
elsewhere. Once they've got the habit of parking elsewhere they probably
won't return for months.

Owain