Yes PRO, or TNA as it is now, really at Kew.
You can do searches online, http://www.nationalarchives.gov.uk/, but not completely comprehensive, search under’Avia’ and ‘Air’ files.
These documents are generally passed on by the AHB, DoRIS at RAFM Hendon may well have some aswel.
Beware some of the specification files are large!