Not sure if this should be in gardening or events, but The National Council for the Conservation of Plants and Gardens has several plant sales coming up for plant-mad folk:
Sunday 23rd April - 10 till 5 - Capel Manor, Enfield
Sunday 6th May - 10:30 - 5 - St. Michael's C of E Primary School, North Road, Highgate.
(See \link{http://www.nccpg.com/Default.Aspx?Page50.Aspx?Page=50\this page} for proper info or to check on plant sales in different regions).
I have no connection to this organisation, and I don't think it's a 'for profit', so I hope this isn't deemed as unfair advertising. The plants for sale are all grown by small independent growers or nurseries and tend to be very choice named specimens etc.
I went to the one in Highgate last year and bought LOADS of fab and rare bits. Really good plants and very helpful to speak to the growers to learn about optimum growing conditions or the benefits of different varieties etc.
Roll on Summer !!! 