You Will Need to get this leak sorted, if you have a combi and a seal system you will find the pressure in the boiler continually dropping, and if you have a open vented heating system, it will leak continually until it is sorted.
The brown substance you have described is a sludge formed in the heating system, it can build up a clog the heating pipes and radiators, you may find some radiators are hot at the top and cold at the bottom where sludge lays when the heating is on.
When you get a plumber out to fix your leaking radiator, ask them to sample your central heating water, you may find there is less sludge than you think and it may be able to be removed with some chemical cleaner and a cold flush or if the build up is quite high, you may need a chemical flush or power flush to remove.
If you have a problem with Sludge i would also recommend installing a magnetic filter on the return pipe to the boiler, doing this will collect any sludge before it enters the boiler, protecting the boiler.
All the Best
The Posh Plumbers