http://www.heathrowairport.com/static/Heathrow/Downloads/PDF/hotel-hoppa-all-terminals.pdf 

You will see that the H10 goes from terminal 3 to the Premier Inn T5.

I stayed in this hotel earlier this year and can recommend the self-service check in, it is very fast.

I am on the same trip and look forward to meeting you. I am staying at the other Premier Inn (Bath Road) on our return and I am parking in the Purple Parking next door.

Not a problem, you can catch the Heathrow express between terminals. There is information about this on the Heathrow website here:

http://www.heathrowairport.com/heathrow-airport-guide/travel-between-terminals#table 

Having arrived at Terminal 3 myself recently I can say that all services from the airport are well signposted.

Kindest regards,

Julia

Sorry to bother you again but this is all new to me

When I arrive back from Thailand into terminal 3 I need to get to terminal 5 to catch the hoppa bus to my overnight hotel (Premier Inn at terminal 5)

When I have reclaimed my luggage at terminal 3 what do I need to do ?

Many thanks

Alan