just checked, you're right, 45A stated on the switch but a 15A fuse in it.
Anyway, the main CU is as follows..
Total Max : 80A
Lights: 5A
Cooker: 30A
Sockets: 30A
Immersion heater : 15A (not used).
So, given that these all wont be on at any one time, can we safely draw a 45A off...