Build and install a recent version of media-tv/tvheadend (3.9 seems to work). It needs to have the SATIP and upnp commits included. This is included in packages.sihnon.net

Start the tvheadend service systemctl start tvheadend

Browse to Configuration→DVB Inputs→TV Adapters. If the right version is installed, it should automatically detect the SAT>IP device via uPnP.

Switch to the Networks tab and click Add

Choose DVB-S, tick Network Discovery, Skip Initial Scan. From Pre-defined Muxes choose 28.2E:Astra for UK Freesat. Don't forget to click the Save button on each tuner!

Switch back to the TV Adapters tab and for each tuner that has a LNB connection, select Position #1 (AA) and tick Enabled, and choose the Freesat network defined earlier. Also select the Tuner itself (parent of Position #1) and enable that.

Switch to the Muxes tab and click Add

Choose the Freesat network created earlier, select Delivery System to DVBS2 for Freesat HD. Select EPG Scan to Force UK Freesat and accept all other defaults.

Wait a few minutes and channels should start appearing in the Services tab. Filter on Encrypted=no to hide the unwatchable entries.

Select the channels that should be exposed to XBMC and click Map Selected. Tick Check Availability, Merfe Same Name and Create provider tags, then map them. This will take a while. Keep an eye on journalctl to see progress.

Restart XBMC to pick up the new plugins

Disable all PVR Client addons except tvheadend

Configure the tvheadend client to set the hostname/username/password etc.

Browse to System→Live TV, set Enabled