[MovedGoalPosts]

OK, looping the aerial:

Take the coaxial out of the STB to the Y splitter
Take the rooftop coax into the Y splitter

From the tail (single) of the Y put the connect to the VCR input (aerial)

From the VCR output connect coax to the TV.

Benefit of connecting the VCR in this way enables you to record terrestrial programmes (BBC1 & 2, ITV Ch4 and Five) whilst watching the STB channel (usiing the SCART connection). Alternatively you can still watch a terrestrial TV programme (coaxial) while recording the STB (scart).

When purchasing a Y splitter make sure that it will allow 2 feeds into one. They were intended as "splitters", not "mergers".

You don't want to loop the coaxial from one appliance to the next to the next too often. You can get drop of signal strength.

As for checking the signal channel being used for coaxial, unplug the Scart leads. and the rooftop aerial from any Y splitter
Connect the STB direct to the TV via coaxial. Tune the TV to the STB's output, as a new telly channel say no6
Disconnect the STB and connect the VCR by coaxial. If the TV gets a picture straight away on channel6 the VCR is on the same channel as the STB. Check your VCR manual for how to change this. Tune the TV to the VCR frequency, as a new channel on your telly say No7.
Unplug the VCR, connect up only the DVD via coaxial. Check that channels 6 & 7 don't get a picture. If one or both do you need to change the output frequncy of the DVD. Tune the DVD to a spare TV channel say 8.

Now you can plug it all back in properly as posted earlier.