Dura Ace is Shimano's top group of components, as used by professionals. Dura Ace part numbers are 7xxx and the 740x series was available between 1985 and 1993. I'm not sure when Shimano introduced 8 speed, probably around '91/'92 which gives you a ballpark date for the bike.
The rear derailleur is actually marked "8 SIS" for "8 [speed] Shimano Integrated Shifting", for use with indexed down tube shifters. If you look elsewhere you'll find the actual model number: 7402.
The 600 series became Ultegra, which is Shimano's second best component group; possibly your Cousin kept the pedals from his previous bike: pedals are quite a personal thing and are often swapped between bikes.
Check the front derailleur, bottom bracket cups, the hubs, the seatpost and the stem: quite likely they are also DA.
"Renovated" on the Reynolds decal, indicates that the bike has been resprayed at some point, quite likely by Bob Jackson as it has proper BJ decals. Unfortunately, there is only one type of "renovated" decal, so we don't know exactly which flavour of 531 it's made from. Quite possibly it is 531ST (Super Touring) which is a more sturdy variant designed to cope with loaded touring; ST was sometimes also used on larger thoroughbred racers and track bikes, where thinner gauge 531 would flex.
If you check under the bottom bracket shell, you should find a serial number: being a custom build Bob Jackson may be able to give you more information on it, such as when it was built and which type of 531 was used.
As to value, the frameset (frame and forks stripped of all components, including headset and bottom bracket) would be worth in the region of £150 quite easily: Bob Jackson are highly regarded. It's true that being a larger frame your market is somewhat limited, but the reduced numbers of frames in such sizes means they are sought after.
I'm a Campagnolo man so couldn't really value the components, except to say that the complete groupset will easily be worth as much as the frame and forks.
For maximum return you should sell the frameset, wheelset and groupset separately; if the hubs are DA, include them with the groupset.
No doubt some will say that it's a shame to split the bike, which it is up to a point; however it's not a classic model (it's a custom build) and you'll appeal to more potential buyers than trying to sell the complete bike.
It's quite likely that if someone bought the bike whole, they would strip it and sell either the frameset or groupset anyway: cyclists are a finicky bunch and have their own very particular ideas as to what they want to ride.
