This art shop in Stockport does cheap Daler Rowney Studland Mountboard. My husband works around that area, so if anyone needs some getting let me know. Aparently she supplies the Stockport photograph people with it and is a main stockest for the boards.
http://www.turnersartshop.co.uk/offers.html
She will cut them to 50 x 40 too in any quantity. So that's 75p per black or white or £1 for coloured mount board at 50cm x 40cm.
Daler Rowney Studland Mountboard. More than 50% off RRP!!
A1 Hi-White & Poster Black - £1.50 a sheet (RRP: £4.15 - 64% discount)
A1 Coloured Mountboard - £2.00 a sheet (RRP: £4.15 - 52% discount)