The places I can recommend for a special crab meal would include: The Ship in Distress, Christchurch; Rick Stein's, Sandbanks; The Pig on the Beach, Studland; the Priory, Wareham; and the Guildhall Tavern, Poole. However, none of these guarantees crab as a season-round main course. If the weather is stormy, potters won't be able to go to sea anyway, so supply of really fresh crab is never guaranteed and it's worth ringing the restaurant to check if they have it, and if reserve if you really want to eat it.
If you're looking further afield, The Crab House Cafe, in Portland, is also pretty good.
Thank you we have now booked a table at the Crab Restaurant. I will post our review after the weekend.