Hi Tony and welcome.This is quite a common problem on the earlier models and has been covered a few times over the years, this is how I did mine it is fairly easy if you can solder.
First off you need to remove the cluster from the car ,then dismantle it as per the Lexls links below ,make sure you follow the first one exactly regarding when to disconnect the battery.
http://www.lexls.com/tutorials/electrical/instrumentcluster.html
http://www.lexls.com/tutorials/electrical/clusterdisassembly.html
You only need to follow the second one to board removal part .
Once you have the boards separated you need to find and replace the following electrolytic capacitors ,it is ok to use ones of slightly higher voltage . They are readily available from places like Maplins ,you will need a small 15 or 20watt iron ,the capacitors are surface mounted to the board but are just ordinary capacitors .
C212 (on the small Board) 10 uf, 16 V
C142 (on the big Board) was 4.7 uf, 25V C147 (on the big board) was 10 uf, 50V
These were board numbers in my Mk2 I assume the numbers would be the same on your Mk1 as the clusters are similar .
Reassembly is just a reversal of removal ,just remember to not connect the battery until everything is plugged back in .
The biggest problem I had was unplugging one of the connectors on the the back of the cluster it was a right cowson and caused much swearing and cursing before it finally gave in .
Best of luck with your car.