Had some interesting feedback from A Raleigh bike forum expert. It would appear that the bike was possibly a custom build for a specific customer - I've been told it could have even been made as a one off special for someone close to the Raleigh sbdu workshop.... Looks like it definitely isn't a mixed bag of parts, which does fall in line with what I know of its history so far.
I'm going to do a bit more investigation into how the family came to own the bike twenty something years ago.
Its amazing how much information people know on these forums!
Having just seen this thread I thought I would add my thoughts...firstly the SBDU only did custom builds - they are all therefore"one-offs".
a mixed bag of parts, the SBDU did not make or sell complete bicycles - framesets only.
I've never seen an SBDU frame with simplex dropouts or mudguard eyes...this doesn't mean it's super valuable so don't get excited! I'll have a look in my SBDU catalogue to see if this specification was listed - the 753 sticker throws me off a bit: mudguard eyes on a 753 frame!
Co-founder of a V-CC section, member South Eastern Road Club and owner of - 2005 Bianchi Reparto Corse (modern), 1985 Raleigh SBDU Pro Super, 1984 Raleigh LU Corsa, 1980 Allin Stan Butler Special, 1978 Ron Cooper, 1975 Bill Philbrook tourer, 2 x very early Roberts (mid/late 1960s), 1966 Raleigh Superbe Roadster, 1964 Allin Stan Butler Special Belgique, 1951 Hobbs of Barbican S/C, 1950 Hobbs of Barbican S/C, 1947 Hetchins Super Special, 1908 Centaur Featherweight. 1988 Specialized Stumpjumper.