Thanks to everyone across the various av forums who have either given me advice directly or indirectly via other postings (freeview, sky, setop box etc). Experienced readers may find this posting rather obvious in places, but there may be other innocents like me reading it. My starting point was one of almost technical ignorance of how the technological components actually worked and worked together. Now my kit works perfectly as a result of your postings.

I bought this wireless AV sender from Maplins:
www.maplin.co.uk/Module.aspx?ModuleNo=224299&doy=5m5&C=SO&U=strat15

This is my configuration:
1)  My main TV is an old 28" CRT (which I will replace when I have to).
2)  A new Humax PVR 9300T which is attached to the CRT AV1 scart connection. A coaxial RF aerial is connected directly to the PVR, and I have an RF loop attached between the CRT and PVR as well.
3)  A new Sony KDL-19L4000 digital TV for the kitchen/bedroom, which normally has no coaxial RF aerial connected to it. This can be on the same or different floor to the PVR.
4)  The Nikkai video sender transmitter is attached to the PVR AV2 connection and the receiver is attached to the Sony AV1 connection.

The video sender instructions for connecting the components together are straight forward. However, the instructions regarding the orientation of the 5.8GHz A/V antenna and the UHF antenna are almost indecipherable. However, once I understood what these gizmos did then it was quite easy to adjust them. In fact I didn't need to. They are all pointing up vertically, with the A/V antenna facing each other.
1)  The 5.8GHz A/V antenna transmits the audio video from the PVR.
2)  The UHF antenna transmits the remote control signal through the walls and floors.

The main problem I had was that the remote controls wouldn't work on the Sony. It took me ages to discover that the video sender's little IR mouse must be placed directly in front of the PVR's IR receiver (I have read that some people tape it to the PVR). As remote controls work via infra red which can't go through walls and floors, the video sender's transmitter acts like the PVR's remote control via IR mouse, whilst the IR mouse communicates back and forth with the PVR.

For the Sony to receive from the PVR, its internal input must be switched to AV1. The Sony's remote control can still be used for TV functions like the menu and volume, but once the Sony is receiving from the PVR, then the PVR's remote control must be used to control it, e.g. for recordings, digital TV and for any of the PVR menu functions. I can even put the PVR in and out of standby from the Sony.

This configuration gives me the following options:-
1)  Both TVs can receive the same digital TV channel or recording, all via the PVR. Note, the Sony does not need a coaxial aerial.
2)  Whilst the Sony receives a digital TV channel or recording via the PVR, the CRT can receive a different TV channel (using its own TV tuner) via the RF loop through, by pressing the TV button on the CRT's remote control. (Of course only analogue on this old CRT.) 
3)  Whilst the CRT receives a digital TV channel or recording via the PVR, the Sony can receive a different digital TV channel (using its own TV tuner) via a 2nd coaxial aerial  plugged directly into the Sony.

One thing I have discovered with the above set up is that the only way I can switch from either TV using its own tuner back to using the PVR is by first switching the set of or to standby, then switching on again.

I live in an old cottage with brick internal walls, BT wireless broadband and naturally a microwave in the kitchen. The Sony TV, with no aerial, even works perfectly when stood directly next to the microwave!

Please feel free to correct me if I have got anything wrong

Once again - THANKS!