You've also the option of T5->Picc Line->Hatton Cross->Picc Line->T4, and in the morning T4->Picc Line->T123->Picc Line->T5.

My experience is that it's no quicker than the bus, but you don't have to cling on to your 4-wheeled luggage quite so tightly.

Don't bother with heavy rail. The frequencies aren't regular enough.

The HIEX at T4 is pleasant and serviceable - it's one of those "split personality" hotels, half is a HIEX, the other is a Crowne Plaza. You can use the restaurant and bar in the Crowne Plaza and charge to your HIEX room.

You will also find this post particularly helpful regarding arriving in one LHR Terminal, and doing an airside transit to exit and cross the border in an alternative terminal:

https://www.flyertalk.com/forum/brit...l#post25935806