When the engine is up to temperature, the heater matrix will always run hot, irrespective of what temp has been selected for the cabin. The matrix is connected straight across the engine block/cylinder head, and so runs at the same coolant temp as they do. (Cool or cold air for the cabin is achieved by the heater flaps altering the air path.)
With the cabin fan switched off (so there is no fan cooling), both pipes at the matrix should feel the same temp (hot). If the fan is switched on, one of the pipes (the return one) will feel just slightly cooler that the other one (supply). If the return pipe feels very much cooler, that suggests coolant flow through the matrix is slow/restricted/blocked. Normally coolant flow through the matrix (provided by the water pump) is quite brisk.
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Appreciate that the inlet pipe to the matrix always runs hot when the engine is up to temp Chris - I meant turning the cabin temperature to the coolest setting cold will prevent any heat from entering the cabin and so ensure the return pipe from the matrix is hotter - which will give a better indication. So if the matrix return pipe is still significantly cool, then it isn't circulating.