I replaced the screen on my 749, the screen screw were like cheese - soft as sh*te. I did the same as you went to halfords couldn't get the correct size, got one at the ally pally show in the end.
Ended up having to junior hacksaw a grove in the screw head so i could unscrew the fastener as some had been rounded off. Worked a treat :-)
Worth getting some new fasteners or getting a set from
www.tastynuts.com (Pro-Bolt) IMHO.