I wouldn’t use the Heathrow Express.
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You need to decide whether a 10 minute time saving (but an extra change) is worth the additional fare. Note that when a friend of mine travelled to the airport on the express he paid £25 each way. It’s not cheap....
Agreed. When I landed into T5 from Berlin a few weeks ago, I got a bus to a GWML station (I think I went to West Drayton, but Hayes & Harlington would also be an option); the bus did not take long and the overall fare paid was very low. The fares from those stations through to Kings Cross St Pancras are entirely on the TfL fare scale (no mixed mode 'tax'!).
I need to get from Leicester to Heathrow (I want to use the Heathrow Express) and a couple of days later to return, but I’m landing at London City airport. Advance tickets don’t seem useful, as I can’t guarantee the landing time of the return flight.
If Advance fares are cheap, there are plenty of pubs in the King's Cross area where you can wait for your train, to allow plenty of time for delays.
My travel times are off peak. My current solution is to buy an off peak return from Leicester to Heathrow, and for the return journey to use contactless to get from London City to St Pancras, and then use the return portion to get back to Leicester.
Questions: is my proposed solution valid? Is there a better solution? The use of the Heathrow Express in non negotiable for various reasons.
If you want flexibility then a return is better, but an Off Peak Return (SVR) from Leicester to Heathrow Rail isn't cheap at £137 (£94 to London Terminals).
Without knowing the dates and times it is difficult to give meaningful advice; for example Heathrow Express do sell cheap Advance fares that are valid on any train on the selected date, which are based on how far ahead you book.
If you can be persuaded to use the Piccadilly Line, then an Off Peak Return from Leicester to Zone U12456 might be cheaper than Leicester to London Terminals, for example if you held a Railcard and if your travel within London was at times that would be considered 'peak' on Oyster/Contactless, then that would definitely be cheaper, but there just isn't enough information to be able to give comprehensive advice.
But if you insist on using HEX, I can't really see anything cheaper than an Off Peak Return to London Terminals, Oyster to Paddington, an expensive ticket to Heathrow, and Oyster/Contactless on the return between City Airport and St Pancras.
Are you definitely travelling at times that require an Off Peak Return, and not Super Off Peak?