Thanks for the messages guys, really appreciate it 
@Mark A most of the cap iron screws will be a standard length, if you can work out the thread and pitch (could be either 1/4"or 5/16") Mcmaster may have something suitable LINK
or if you go down the re-threading route then have a search for "cheese head screws" on e-bay, you just need to make sure the head diameter is large enough.
E-bay link
Workshop heaven list a long Clifton screw
workshopheaven.com
as do Thomas flinn
www.flinn-garlick-saws.co.uk
Hopefully these will be long enough.
@Mark A most of the cap iron screws will be a standard length, if you can work out the thread and pitch (could be either 1/4"or 5/16") Mcmaster may have something suitable LINK
or if you go down the re-threading route then have a search for "cheese head screws" on e-bay, you just need to make sure the head diameter is large enough.
E-bay link
Workshop heaven list a long Clifton screw
Clifton Extra Long Cap Iron Screw
Clifton Extra Long Cap Iron Screw for Clifton hand planes British-made Buy woodworking tools online at Workshop Heaven. Next day delivery. Lifetime guarantee.
workshopheaven.com
as do Thomas flinn
Clifton Cap Iron Screw - Thomas Flinn & Co Ltd.
Spare screws for our cap irons. Longer thread to be able to take our thicker plane blade and cap irons.
www.flinn-garlick-saws.co.uk
Hopefully these will be long enough.