depends on the CU, if its well wired and in good condition then its perfectly safe, if shrouds are missing or the tails are badly terminated then it can be somewhat more dangerous. Always take great care not to touch anything the first time you take the lid off a CU and then have a good look at the area on the supply side of the switch to see if there is any exposed live metal.
The difference in disconnection times in the event of a short circuit between a 20A and 32A MCB is negligible and there shouldn't be any problems with cables overheating as long as the cables were suitably sized in the first place.
I want to reduce the old MCBs from 32A to 20A - Garage Shed, security lights, and remove a 5 amp bell MCB - not used and replace with cover.
Are you saying your security lights are on a dedicated 32A circuit or that your security lights are run off the 32A shed circuit?
Depending on your answers I really don't think there is any need for you to mess with the consumer unit. Assuming everything is working (and to a safe standard) and you don't need the space taken up by the bell transformer just leave it alone. You just run the risk of making a problem.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local