We use Transmanche all the time-usually once every 6 weeks and have never had any problems at all, in fact we find they tend to bend over backwards to help, even phoning to let you know your sailing has changed to a later time.
Yes they are a "no frills" operator and the ships are basic (at the moment-2 new builds are due next year) compared to others but we like that and it seems to make the crossing more relaxing. The food on board is good and cheap and the crew helpful and friendly.
TM will allow you to sleep on the dock at Dieppe, amongst the trucks as long as you e-mail them first, we've done it on numerous occasions, and you can use the freight "facilities".
If you miss the sailing don't worry ring them and they'll put you on the next.
As you can tell we're fans of TM so if you've got any other questions let us know, oh and the good bit, discovery bailey and 3 adults in Aug for 18 days