Just an update, that worked perfectly as you (and others) have said previously.Not quite, you need to subscribe to an active plan, then you can move. I subscribed to weekly first, then switched to monthly
Basically I subscribed to the weekly option first, (£3.99 per week) when I went into the app to change the subscription plan I then saw the monthly option for £5.99 per month. I went to subscribe / upgrade to the monthly plan and it said that Apple will refund any of my unused previous subscription. Few hours later I got the £3.99 refund back and ended up with the £5.99 per month which is what I wanted.
Find it strange that the option is not available up front and I have to go through this convoluted way to get to the monthly plan.
Thanks all for the advice!