Google Maps found this one just round the corner which seems to be open till 6pm on Sundays:

www.avis.co.uk/drive-avis/car-hire-locations/europ...n

There may be others.

Thanks for all the tips. One of the Heathrow terminals might be the favourite option, with their easy access to the M4. I need to look at overall journey times.

I suspect that's going to be your best bet.

The place in Eversholt St is near enough that it could be branded Kings Cross/St P but it's only open until 18:00. Wouldn't need much of a delay in the train plus arrivals hiatus at St P for that to be a marginal call.

St P to Marble Arch by tube involves a change or a walk from Hyde Park. As you say staying on tube all way to LHR puts you on right side of London for M4. Flip side is LHR geography. The tube will take you to the terminals but the hire car pounds are on the perimeter road - a bus transfer.

Avis Eversholt street is literally at the side of the station, i wonder if they have a coded key safe, you could pre book a car which would be left out for you in the event, worth phoning up, its got to be the nearest well known rental site.

If not then they would know of any other central town Avis sites that might have later hours should the train be delayed, eg Mayfair office which is just behind the old US Embassy at Grosvenor Square, a short cab or tube ride.

I suppose one worthwhile benefit of getting out to Heathrow rental desks is you won't be driving out of London itself at peak time, ah Sunday no peak time worries.

Edited by gordonbennet on 18/01/2020 at 18:54

It takes best part of an hour to drive to Heathrow from St Pancras even when the roads are clear, and about the same on the tube. However, picking up a hire car from most of the hire car operators Heathrow is not particularly quick. Most operate from a "Car Rental Village" which is out near the long stay car park, and annoyingly each hire car operator has their own minibus which doesn't run that frequently. I reckon it takes about 45 minutes on average from getting to Arrivals to driving off in the hire car (and that's for a corporate hire where they pretty much just give me the keys.

There is one car hire operator who is at Hatton Cross Tube Station (the last station on the Piccadilly Line before the airport) instead, I think it's Sixt. That would be much quicker from the pick-up time perspective, particularly if you are coming from London rather than arriving on a plane.