He's got a 24kW water heater now so it should be simple enough to measure. Idon't remember about the OP's cooker and/or fire position, but remember you're expected to leave room for some additional load.
I've measured long runs of gas pipe with all joints very carefully, completely deburred, with long radius elbows, and always found the tables to be a long way out. One I remember, 28mm at the limit on length, gave nearly double the predicted drop.
Existing pipework is usually put in with far less care, dirty etc, and can give 3 times the drop in the tables. One blob of green growth from using the wrong type of flux and you're stuffed.
Recently I measured a 2mbar drop in a short length of iron which should have been about 0.5 mbar, so I blew it out with a vac (DIY'ers DON'T do this, you're liable to blow yourself up!) and got it down to 1mbar.
I'll bet a pound that he OP's installation would be undersized if it's all 22mm!