7,300 Watts supplied on a 2.5mm2 T&E.

Let's assume that normally the heater will be used to ⅔ fill a 15 l washing up bowl at 50 deg C with the incoming water at 15 deg C. How long will it take?

10,000 grams of water are to be heated by 35 deg C, which requires 10,000 x 35 x 4.2 joules = 1.47 MJ. The heater provides 7,300 joules/sec. So the time required is 200 seconds or nearly 3½ minutes.

The circuit could be made compliant by fitting a B20 MCB. Would that do?

Yes, according to Figure 3A4 - it would trip only after 500 seconds.

For me it's a C2 because the cable could be overloaded if, e.g. the heater were inadvertently left running with the plug out. No fault to danger! My recommended remedy would be to downsize the MCB to 20 A. There should be no nuisance tripping given the intended use. (Or 25 A if RM C.)

Let's assume that normally the heater will be used to ⅔ fill a 15 l washing up bowl at 50 deg C with the incoming water at 15 deg C. How long will it take?

10,000 grams of water are to be heated by 35 deg C, which requires 10,000 x 35 x 4.2 joules = 1.47 MJ. The heater provides 7,300 joules/sec. So the time required is 200 seconds or nearly 3½ minutes.

The circuit could be made compliant by fitting a B20 MCB. Would that do?

Yes, according to Figure 3A4 - it would trip only after 500 seconds.

For me it's a C2 because the cable could be overloaded if, e.g. the heater were inadvertently left running with the plug out. No fault to danger! My recommended remedy would be to downsize the MCB to 20 A. There should be no nuisance tripping given the intended use. (Or 25 A if RM C.)