In case it helps anyone, I've finally resolved this issue.
After installs of several operating systems, two replacement cards, changing the PSU unit, installing a brand new satellite dish + cables + Technomate Quad LNB and also trying a standalone Freesat receiver the problem eventually turned out to be a faulty PCI-E x 1 slot on my motherboard.
I'd never used this slot on my motherboard before installing the TBS6981 so had no reason to think on a two year old system that the PCI-E x 1 slot had anything at all wrong with it.
I've installed a new motherboard and scanning works perfectly. Previously it would perform a single scan of one transport (straight after a cold boot) and then failed with the following errors messages:
- Code: Select All Code
WARNING: filter timeout pid 0x0011
WARNING: filter timeout pid 0x0000
WARNING: filter timeout pid 0x0010
MythTV is now able to scan successfully for channels using the satellite card.
I hope this helps someone else...