"The HDMI from the DVD/Surround will be transmitting the picture from the DVD/Surround to the TV. Does sound come out the TV speakers when you play a DVD, or is it only from the speakers connected to the DVD/Surround amp?"

Only through the dvd 5.1 speakers

Okay good, that means the HDMI is transmitting the picture ONLY to the TV and not audio aswell. This is what we want....
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"However, if you rarely use the TV's internal tuner (which is most likely either analogue ch1-5 or Freeview) and only watch Sky, then just connect the red+white RCA's from the Sky box to the DVD/Surround."

Assuming they are inputs on the dvd player? if so how to connect to the tv after being fed to the dvd?

If they are inputs on the DVD/Surround yes. But you don't need to connect to the TV after connecting the Sky box to the DVD/Surround - the Sky box will feed picture and audio to the TV through the scart, and then also send audio to the DVD/Surround through the red+white audio cables. So you should get sound from both the TV and the DVD/Surround is hooked up like this, and both turned on (DVD/Surround will need to be set to the AUX or whatever input channel)
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"I take it the Sky is connected to the TV using Scart? If so then you will get audio through the TV speakers from the Sky box over the scart cable,"

Yes it is.

See above.....
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"Does that all make sense? "

Nope not one fookin bit lol

Good