Here is my current setup:
Satellite Dish Fitted by sky - goes to splitter (one to sky box) (one to tv card on PC for DVB-S Freeview) (Signal on tv card and sky box almost perfect in strength and quality)
VHF aerial ( large on high mast) - goes to sky box. (signal is a bit ghosty on some channels TV1 & TV3 mainly but does not deteriorate between the 4 tv's)

RF out from sky box goes to 4 way splitter going to 4 separate TV's 
Composite out from sky box goes to main 40" LCD


I am looking at adding a UHF aerial to get DVB-T FreeviewHD

1. I am unsure what the signal will be like as the VHF reception isnt that great although is watchable with big aerial on high mast. Also unsure whether it is worth getting a big High Gain 91 element aerial like this one
Or whether a 43 element one would do.

2. I was planning on putting the UHF aerial about 50cm below the big VHF aerial on the same mast as it would be pretty difficult to put the UHF above the VHF as the mast is pretty tall already 1.5m above the roof at least.

3. I was planning on combining the UHF and VHF signals in a diplexer on the roof before going down to the sky box in the lounge then taking all tv's from the 4 way splitter ( from the rf out of sky box see above). Will all tv's get the current sky channel plus all vhf and uhf frequencies?

4. The Hybrid DVB-S and DVB-T + Analogue TV Card is a room on the other side of the house to the sky box. So i would be taking the cable from the splitter box to the other side of the house to the PC. But i already have a cable from the sat dish to that room and was wondering if it was worth combining the sat and vhf/uhf/sky signal in the one cable with a sat terrestrial diplexer then splitting them again at the other end. (Will this deteriorate the signal further? Is it worth it should i just run 2 cables?)

Thanks for the help!