I've had a read-up Jo. It seems to be quite possible to de-scale and de-sludge the exchanger either in situ, or by removing it. Results are reported to be very good.
One thing I must add though, is that there is some disagreement as to whether this is a DIY job. Some GasSafes say go ahead and flush, but others maintain that since gas connections are involved, it must be done by a GasSafe.
Use your own discretion here. If you can flush in situ without touching any gas connections, then try it.
As with so many things, it's a grey area as to whether it should be done by a GasSafe.