I know my heat exchanger has scale. Is it likely to be the main exchanger or the DHW.

The most common cause of kettling is limescale build up in the main heat exchanger, this commonly occurs in hard water areas, the heat exchanger could be replaced or cleaned but if your boiler is over 10 years old I would recommend replacing the boiler and also fitting a magnetic filter and a hard water filter (if you live in a hard water area) to reduce the risk of the problem reoccurring in the future.

Good morning.... The fault could be with the main heat exchanger the pump or the system is dirty and requires a power flush

Limescale does not affect main Heat Exchangers. It’s more likely to be sludge deposits of restricted filters within the boiler, depending what boiler you have.

There is not a simple answer to this question, The engineer that attend this breakdown would acess is it is a kettling noise or a hammering noise. If it is a kettling noise it would be worthy to check the waterways of the condensate. Condensate supmp, all pipe of the condensate inside boiler, condensate trap and condensate pipe. If it is a hammering noise, it may be possible to clean it with the powerflushing machine, if it is a one-pipe-in and one-pipe-out main heat exchanger. However if it is main heat exchanger built with a manifold and several pipes in parallel, then it is pointless to use the powerflushing machine because the cleaning product would circulate freely through those pipes that are not blocked. It is beneficial in any case to install a magnetic filter after the repair/replace of the main heat exchanger, and it is necessary in order to learn what caused the problem,so it will not break the new main heat exchanger, to examine the type of dirtiness that your system have, is it black colour ?, that would be ferrotite sludge, and the best course of action would be to powerflush the system and treat it with corrosion and scale inhibitors, before installing the new main heat exchanger. Is it a red-orange colour sludge ?. In that case the problem may be due to constant fresh water entering in the system, for example a gravity system with header F&E cistern, indication that the system is losing water, and this have to be repaired otherwise in time the new heat exchanger will break as well.