The seal has been damaged, and a new one can be bought for about £50 (!). Is fitting a new seal a DIY job? If not, how would, say, the cost of the seal plus professional fitting compare with buying a new door? Thanks for any advice.
Lift the inner side of the seal and see if there are any screws retaining it. If there are none, it’s a bonded door, and your only recourse is to buy the door complete. Even allowing for the seal being listed as a spare, you’ll never fit it.