@ aysyr & Gridcoin -  thanks for the replies.

This may be a face-palm moment but I don't have any GRC in my wallet. For some reason  I thought the beacon would loop the payment back on itself and so not require a balance. Please can you/someone send me enough GRC to get started 

GG6XHgjJYmzaSxAeRTeRRhC1g3Smgk1QDh

If this fixes the issue 'll add a post to the 'How to CPU mine' thread as I think the current explanations miss this out.

I have Gridcoin v1.1.5.7-g71-prod

Thanks for your help!