@Gertjan
So an update, after a week of issues I've narrowed this down to pfBlockerNG-devel/DNS issues. I've removed pfBlockerNG-devel and reverted to AdGuard Home and I've had a stable WAN connection and box for 3 days. I did notice this and wasn't sure if that was a contributing factor:
https://www.reddit.com/r/PFSENSE/comments/88wg6g/issue_with_pfblockerng_dnsbl_and_cloudflares_1111/
@Rico - I made these changes and it made no difference when DNS was down or been blocked by pfBlockerNG-devel
So thanks all for the help and pointers, all really useful and I defiantly improved my knowledge of PFSense and DNS over the last week!