Here is my 0.02 cents on the dorking as they are this moment. Not worth anything to a farmer. Well that goes for most heritage breeds these days.
If Dorking,NH,RIR,RIW, have any chance at becoming and fullfiling what you think is a dual purpose. We need at least one breeder is able to devoite time, engergy and money to the breed.
Start off from every line you can get ahold of. I don't care how closely related they are! Because every one breeds differently.And it does not matter where or who they come from. Be it hatchery, farm, private breeder. Out of all those keep details on where they cam from, how fast they are growing, the weight they put on etc. Then when you select your breeders look carefully at all of them! For the dorking, choose ones with a deep keel and dense breasts. Amoung other things (of coarse). And don't just hatch 10,30,50 off hatch 150,200 off then you well be able to tell a portion of what makes up your flocks genetics. You don't have to hatch them are raise them off all at one time. Select for egg size. Skin quaility, feathering. Measure, weigh, tag !