I can recommend Jacksons - but they don't have a wide range of stuff, but still worth a look. Reasonable prices, best packed plants I've ever had.

What plant are you looking for? You forgot to mention them.

Burncoose and Longacre plants are both good as is Cotswold cottage garden. they are not cheap but they all produce good quaity plants.

Pottertons

If you go to the RHS website (www.rhs.org.uk), select 'Plants' then 'Plant Finder', you can find who stocks the plants you are looking for.

is it not worth you looking around for one of these huge garden centres , find out if they have what you want and make a whole day of it . i just think with a lot of plants your better of choosing them personaly realy .sometimes you can get public transport but obviously easier with a car . maybe you have some gardening friends who will chip in on the petrol with a nice big estate car . andrewr has a good place to start . wear theres a demand theres nearly always a suply . the trouble ive found is a lot of amazing garden centres are tucked away and you can drive right past them . theres a place i go to that sell tropical type plants in norwich but you would never just drive past it infact you could well drive past it if you had an idea wear it is .

Thanks for all the ideas - we'll get our list and have a look at the RHS website this weekend. Noseypotter, I love your idea, but with two young boys, I've tried pottering around a garden centre and it's a nightmare. Even if I let them choose plants for themselves. Last time we had been there literally 2 minutes, and my youngest was saying 'We've been here all day mummy - when can we go?' So, I'm going for the online option instead.
Thanks x

fare enough i understand .

Makes far more sense Plumhues and if you are looking for specific plants you will at least be able to source them! Most GCs carry a, relatively, boring range which they know will sell; if you want something different you need to either go to a specialist nursery or on-line.