In fact it would seem that the digital multiplexes from the Midhurst transmitter should all be receivable on a 'normal' C/D group horizontally polarised UHF TV antenna..... So all you should need is a good quality, well-installed C/D group aerial; there should be no need to have to replace it with a W group aerial.
Try
http://www.dtg.org.uk/retailer/coverage.html and enter your postcode where indicated. That should give you an indication of which channels you should be able to receive at your location. E.g, from Midhurst, all these Freeview channels should be available:
TV
1 BBC ONE
2 BBC TWO
3 ITV 1
4 Channel 4
5 five
6 ITV 2
7 BBC THREE
9 Teletext
10 BBC FOUR
11 Sky Travel
12 UK History
16 QVC
17 TV Travel Shop
18 The Hits
19 UK Bright Ideas
20 f tn
21 TMF
23 bid-up.tv
30 CBBC
31 CBeebies
40 BBC News 24
41 ITV News Channel
42 Sky News
43 Sky Sports News
45 BBC PARLIAMENT YES
46 Community Channel
50 Four Text
51 BBCi
52 ntl i
RADIO
70 BBC Radio 1
71 1Xtra BBC
72 BBC Radio 2
73 BBC Radio 3
74 BBC Radio 4
75 Radio 5 Live
76 BBC Five Live Sports Extra
77 BBC 6 Music
78 BBC 7
79 BBC Asian Network
80 BBC World Service
81 Kiss
82 Kerrang!
83 Smash Hits!
84 Jazz FM
85 Oneword