Usually to try and clear a blocked matrix you need to remove the heater pipes in the engine bay connect a hose pipe to one of the copper pipes sticking out of the bulkhead and flush it out, then reverse the hose pipe and flush the opposite way, hopefully lots of mucky crud will come out and job is usually jobbed
Right was speaking about cars in general but as you will see from the next thread rover 75 pipes don't allow you to get at the clips at the bulkhead.