Just in case this is due to weather conditions - wind and heavy rain etc.........
On a good weather day - remove the aerial connection from your TV and do a scan for digital channels.
With no aerial connection it shouldn't find any so your tuner will be blank.
Then use a direct aerial co-axial connection from the main aerial DIRECT to the TV aerial connection
(i.e. No daisy chain links in circuit), and do a channel rescan.
This should then find the optimum channels in your area including the elusive Dave on Channel 19.
If you are still missing any channels from the various muxes I would try a TV booster (the one outlet type for highest gain) to see if that gives any improvement.
http://www.dtg.org.uk/retailer/dtt_channels.html
http://www.maxview.co.uk/acatalog/TV_AMPLIFIERS_&_FILTERS.html
Note that Dave is on Multiplex C which includes Sky News (82) Sky Three (11) and E4+1 (30) so if you are getting the latter channels you should also get Dave.