I think you are on a winner, Sanne.
In the past, we've tried the "Heathrow hotel" options, with several variations on the shuttles and buses.
For me, none of them worked well, in terms of the standard of the hotels or the messing about and unreliability of services.
In more recent times, we've used the Sofitel on five or six occasions and it has always worked well for us.
The added benefits, although you may not need all of them on this visit, are the excellent restaurant, the invariably efficient and welcoming check in service, and a good breakfast offering, again with good service.
Then, on departure, you are not at the mercy of bus timetables or car services.
It certainly won't be the cheapest option and the convenience level is not brilliant for T3-T5-T3 but I would still rate it the best available.
Good luck with it all.
Thank you for your advise.
We also have stayed at the Sofitel T5 on several occasions but always when we've been in or out of T5. It's a great hotel both for convenience and service. Only issue is, that this is rather expensive for what for us this time will only be a 8-9 hour transit plus we have to go in and out of T3 which is why I was asking others for their opinions.
Lots to think about. Gone are the days when I picked a hotel from the selection in the brochure and said 'that will do"!! Maybe I just have too much time to make decisions these days.