[Hampshire] [OT] DVB-T channels

I have just taken delivery of a portable dvb-t TV. When I auto-scanned
for channels it only picked up 29 of the available programmes. On
investigation it CAN read:

The BBC multiplex (channel 23 here), Smile/E4+1/Sky3 etc (ch 26) and
Five* (ch 30).

It CAN'T read:

The ITV multiplex (ch 28) BBC4 (ch 32) and Film4 (33) are ignored.

When I try manual scan it shows good signal strength on all those
missing multiplexes (68-70% which is actually higher than the 61% for
the BBC multiplex) but the 'Quality' is shown as 0% (compared with
99% for BBC) presumably indicating it can't decode the stream or
similar.

We have a DVB-T recorder (Topfield) which gets all the programmes with
no problem.

Can anyone think of a reason why the new TV might have trouble with
those multiplexes? Are they a slightly different DVB standard or
anything?

Any help would be gratefully received!

TIA, Peter