molgrips - how much is a HRM? Not £900.
Clearly. But you did say powermeter.
If you are serious about getting fit, a Powermeter is going to yield greater benefits than a set of carbon wheels or light set of forks. Of course a PM is not necessary but a helpful addition.
Those things aren't the same type of spend at all. The OP didn't come on and ask how best to spend £900. Seems to me that he's a relative beginner and hence wouldn't really be keen or dedicated enough to spend hours of self-immolation staring at a few numbers.
First step - get riding more; second step - do base in the winter and speed in the summer; third step - get a HRM and a book; fourth step - get a coach; fifth step - get a power meter, I reckon.