Having never done anything like this before I thought that I would start small and have bought some mixed red lettuce leaves and some watercress to grow in pots. I have bought 10" pots as recomended on packet and thought it might be best to wait till the night frosts finish before planting. Can they be started off on a windowsill and planted out later? If all goes well I might advance to tomatoes and peppers.:D

lettuce is quite tender ... if you are planning to grow it outdoors with no protection, I would suggest waiting until later in the season, otherwise you may be disappointed. Gardeners get itchy fingers when it comes to planting out and most of us chance it during May, but they do say that frosts can occur right to the end of May (in fact I remember a couple of years when we had snow on 26 May). However, if you have a sunny windowsill to keep them on, give it a go.

I agree
I would just like to mention though, peppers need a long growing season, so if you're wanting to grow them, don't wait too long. They will be quite happy on the windowsill.

Having never done anything like this before I thought that I would start small and have bought some mixed red lettuce leaves and some watercress to grow in pots. I have bought 10" pots as recomended on packet and thought it might be best to wait till the night frosts finish before planting. Can they be started off on a windowsill and planted out later? If all goes well I might advance to tomatoes and peppers.:D

Welcome to our gardening section of the forum if you have any problems just post a question and one of us will try to help you best we can.
Now back to your original question many places sell vegetable seedlings at this time of year also you might find this useful
http://www.bbc.co.uk/gardening/digin/

yeah I was presuming you'd buy tomato / pepper plants from a garden centre or somewhere

Thanks I think that I will start my lettuce off on a window ledge it is the cut and come again type that I have. I can't really go mad in the garden as we have a caravan and I am not here all the time so it will have to be something small or low maintainance. I thought some pots on the patio might be easier for a neighbour to water for me if we go for for longer than a weekend.I got watercress as well because I love that but when you buy it in bags it doesnt last long so fancied giving it ago from seed. I am going to get some peppers as well as Hayley 1 said they do Ok on a windowsill, so more of a salad plot than veg!!