What rear suspension do you have?
For a non rotoflex rear:-
1. Jack up rear high enough to get confortably under and put on axle stands
2. From inside remove spring clamp coverplate from rear axle tunnel
3. Remove all spring clamping bolts/studs from top of diff
4. jack up rear wheels so spring lifts off top of diff (may need a tap with a hammer to free up)
5. Get underneath and slide/manouvere spacer block between top of diff and underside of spring.
6. Go indoors and put plasters over your skinned knuckles
7. line up bolt holes of diff/spacer/spring
8. Swear a lot
9. get friend to help jiggle spring to line up bolt holes of diff/spacer/spring
10. Get at least two studs/bolts located through spring and spacer into diff
11. let down jacked up wheels and pray spring and spacer slot nicely onto diff
12. Fit remaining two stud/bolts and tighten down all four
13. Jack down car and roll backward and forward a bit to settle suspension
14. Replace knuckle plasters and have beer
15. Think about getting rear tracking set as now it sits lower you may have a bit more toe-in than before.
16. Drive round corners quicker

(after beer has disipated)
Easy peasy
(If you have a non-roto rear consider re drilling the rear uprights lower for a simple and cheap alternatice to a spacer)
Andy
PS Castle Combe Racing was REALLY good - Sell you children/mother/wife/house/porn collection and take up racing, It really is worth it. You don't have
to be rich, in fact once racing you won't be so why worry about it