The project was set up to digitise BSBI's botanical publications and make them available freely to everyone. If you can't find the paper you're looking for, please try the Biodiversity Heritage Library or one of the other sites listed below.
Douglas Simpson’s comprehensive listing of sources of botanical information relating to the British Isles is now a scarce and valuable tome. It was scanned and digitised by Tim Rich and is made available here as a download. Abbreviations have been expanded to make it more readable.
Simpson, N.D. 1960 A Bibliographical Index of the British Flora. Privately printed, Bournemouth. Digitised and expanded by T.C.G. Rich, 2004.
On the BSBI Literature Database you can search for botanical literature by genus. It is fairly comprehensive for publications on British & Irish plants in the second half of the 20th century.