Heathrow to Euston Station (Bloomsbury)??

Just to point out that it's cheaper to take the Heathrow CONNECT, rather than the Express. It takes about 15 minutes longer, but goes from the same platform and is considerably cheaper. But check times - on Sundays they are not always so frequent, and may stop earlier.

Taxis are very expensive in London and there is a supplementary evening and weekend charge. You can take the Underground (Subway) Piccadilly Line direct from Heathrow to Russell Square Station. This will be very close to your hotel
www.tfl.gov.uk

You won't get a direct tube from Paddington to Euston, only to Euston Square, which is a block away. The distance is actually walkable if you know where you're going and feel energetic, so I would suggest that a taxi won't cost an unreasonable amount, especially shared between two. It will also save stairs - to get direct from Paddington to Euston Square without changing, you need to go from platform 16 in the main station, NOT from where all the Underground signs will direct you to. That will mean going up and down stairs, as will leaving the station at Euston Square as there are no lifts from the eastbound platform there. No - for this distance, it would be reasonable to treat yourselves to a cab. A taxi the whole way from Heathrow, however, WILL be very expensive.

The hotel knows how late you're arriving, doesn't it? Just that you won't get there until at least 9 pm, and my experience of business travel around Britain is that many hotels will assume you aren't coming and release the room earlier than that, so we always used to put in a request to hold the room for late arrival. Which they will happily do for free.

Enjoy London!