OK I had a play with some settings and did the following:

• Changed the subnet mask of the router from 255.255.255.0 to 255.255.254.0;
• Changed the DHCP IP range upper address from 10.10.10.250 to 10.10.11.250;

This worked nicely and devices could communicate fine across the two subnets, however there was a few issues:

• My Kodi boxes seemingly were not able to play files from my NAS after the change, despite no IP addresses changing at all, they were able to navigate SMB shares and playback files directly but not when played from the library? Something to have a play with I think.
• Sky Q lost the connection between the primary box and the mini box we have elsewhere in the house, not sure what this is about but I changed the Dec settings back to thieroriginal settings and rebooted the mini box and it worked fine after;

I suspect what I want to do will be fine but I need to do a full restart of ALL devices on the network (NAS, sky boxes etc...) to rule out any odd behaviour??

Cheers all.