Ah, not sure if you just added how much you get paid for, say, an early onto the base salary. It doesn’t work like that. The base salary includes all your booked shifts so you get no extra money for working a late or an early. For nights the only extra money on top of base you get is the night premium. The only extras you get are if you work your rest days (125%), work Sundays (150%) or nights (whatever night premium for your grade). There’s bank holidays as well to consider. At least that’s how it works for me (I’m relief so don’t have a fixed roster as such, just booked rest days). Apologies if I’ve misunderstood how you arrived at the £47k.
I think adding 10% onto base salary is a realistic, possibly slightly conservative way to arrive at what you might get. I’ve certainly been guilty of having a heavy overtime month and getting used to the fat payslip, only to be shocked when it drops back to the normal level! The overtime is nice but it really doesn’t do to either financially plan around it or rely on it.
You could do £40k as a grade 3 I reckon, depending on how much overtime is available at your location. You would have to smash a few rest days on to achieve that though. As ever, take everything I say with a large grain of salt as I only have experience of a small set of locations and have been doing the job for less than a year.