I'm surprised that in seven pages no-one has mentioned that the six DTT multiplexes (in the UK) are not equal. Also, within one multiplex, the bit rate for "channels" can differ- as can the picture resolution.
Importantly, the actual transmitted power varies- hereabouts, the BBC Muxes are 10kW while the ITV/C4 Mux is (I think) 6kW.
However, most importantly, the ITV/C4 Mux is encoded using 64 QAM in an attempt to cram four times as much info into the channel, c.f. 16QAM for BBC.
So ITV/C4 is difficult to receive at adequate s/n, (as is C5 but because it is usually transmitted at low power). Your IDTV (or STB) is doing a better job with a marginal signal than the DTT adaptor at your location!
P.S. Believe it or not, the mini magnetic aerial is intended for mobile reception- cars being steel while furniture is not in general. The signal strength to achieve this (at up to 100 km/h) is available in major conurbations in Germany. It has been trialled in the UK.