Having decided to take the simpler option and buy an Aston Simba HD (Fransat version) to use with my motorised dish, I discovered that Humax had recently released a TNTSat box, the TN5000HD. I had been a bit nervous about buying the Aston because I had read in a couple of French forums about software problems, freezing, etc so I took the plunge and went for the Humax instead, even though I had not been able to find many reviews on it.
VERY pleased with it, it is certainly not the cheapest way to get French TV in the UK (I paid 219€, plus you will need a separate hard disk drive to record or pause live TV) but you can use it as your main box to receive channels from other satellites. It only has a card slot, not a CAM slot, but it works very well and has an Ethernet port which hopefully will come in useful one day. You are supposed to be able to use this to watch YouTube, but this is the only bit which I have not got to work yet, maybe it is configured to block non-French IP addresses.
I don't have an HD telly but the picture quality on HD and SD channels seems excellent.