Don't the HRMs have a fitness rating to adjust for this effect?
Yes. Mine does.
On MFP, I tend to "round down" to the speed bracket below my actual speed (eg if I rode at 17mph, record it as 14-16mph on MFP). Funnily enough, this seems to give a similar figure to my HRM in most cases. And given that my weight change patterns seem to be reasonably closely in line with what MFP predicts, that suggests I'm getting fairly accurate figures from both MFP and my HRM.
It may be affected by other factors, though, and may not work out the same for everyone.
Auntie H, the way I see it, you have three options:
1. Ride faster. This is what I've been doing. Although this might spoil your enjoyment of your talking book.
2. Ride up some big hills. Hmmm... you live in Norfolk, don't you?
3. Ride a heavier bike at the same speed. Or carry a brick or two in your panniers.
Another option is to vary your exercise regime - I've started doing a bit of running because although I'm "cycling fit", I'm definitely not "running fit", so I reckon I get more exercise benefit from running.
d.