tubechallenge.com

Something I've noticed is that, if you have to miss out the Richmond branch in order to get to HT4 before in closes, it should still be possible to get back to Gunnersbury in order to make the last train to Richmond. Though the last tube-only connnection is

Piccadilly
HT5 (d) 00:00
Turnham Green (a) 00:30

District
Turnham Green (d) 00:35
Richmond (a) 00:44

I think it's also possible to do it later using the N9

Piccadilly
HT5 (d) 00:18
Hounslow West (a) 00:28

Bus N9
Hounslow West (d) 00:36
Gunnersbury (a) 00:52

District
Gunnersbury (d) 00:59
Richmond (a) 01:05

yes it would give you a finish but if your planning to finish Heathrow you are likley to be heading west from hammersmith about 22.10 - 22.30 if your chaseing the record.

The question is, after spending 17 hrs + on the network, after changed a route many many times due to network issues, would you remember to even check to see if this is possible.

I suggest (from experience) not!