It is a feature of television signals that they can be effected by the weather. Aside from the obvious damaged that can be caused by wind or torrential rain, the onset of summer can cause a more intriguing problem called "inversion".
Basically, when the sun heats up the land, the hot air can get trapped below cold air at high level. This creates an "inversion layer" which acts like a polished mirror to TV signals. This causes two problems: you can receive both the original and a reflected signal from your own transmitter (on analogue TV this appears as "ghosting") or interference from TV transmitters that are normally too far distant.
The ITV/C4 multiplex of channels (ITV-1, ITV-2, ITV-3, ITV-4, CITV, C4, More4, E4, Quizcall) use a transmission "mode" called 64QAM, and it is sadly much more prone to these problems.
So, whilst a loft-mounted aerial will provide reception at some time, it will fail at others. The only real solution is to mount the aerial (or a new Class I aerial) outside on a pole, but some people report that changing the cable from the aerial to the set-top box to the high-grade satellite cable can also help.
The SCART connection, by the way, only carries a single TV picture, not the Freeview transmission multiplexes, so if this is disconnected it will result in degradation to every channel at once.
Eve : As both channels are being received OK in another room, then the problem is with the TV.
The best way of getting over the problem is by carrying out a "reset" the TV, this on some brands being referred to as "default setting" or "first time installation", the main point being that everything stored in the TV tuners memory has to be cleared followed by carrying out a normal retune.
Although the aforementioned is almost guaranteed to rectify the problem, sometimes success can also be achieved by removing the aerial connector from the TV followed by carrying out a retune, this also deleting all stored in the tuners memory, checking it has by "reconnecting the aerial" and ensuring that selecting any channel results in "no signal received" message. The final stage being to carry out an auto-tune to load the channels back into the tuners memory.
marion: Ok - first thing - does your TV just have a scart, or does it also have three little plugs, one yellow, one red and one white? These are called RCA's, and kind of do the same sort of thing as scarts..ish. You can get a convertor for these to make them into a scart (kind of) for a couple of pounds, which should inlcude the cables.
Assuming you just have a scart, some digibox's do have the way to connect a VCR to them, etc, but you'd have to look in the manual. Frankly, the easiest thing to do is get those scart switch boxes. You might even get one from a Pound shop, which look a bit like this Bandridge 2 Way Scart Switch Box | Maplin
The one with the buttons should be about a tenner (which I have), but you could get a basic version for less than a fiver on Amazon. Dont spend too much money - your TV, VCR and digibox are all going to be replaced at some point by a new TV, etc, and there is no point spending a lot of money on a temp measure.
marion: If the VCR has two scarts then you might find that feeding the digibox into it and then the VCR into the TV works. Obviously the VCR will need to be powered on at the wall in order for the digibox picture to pass through it. The instructions for the VCR might detail different set-ups, including one where you feed a "decoder" into it, which is what you digibox is.
Chris Blackie : Does it have manual tuning? If so, try a manual tune for the frequencies of your transmitter.
If your aerial points to Craigkelly, to the north of the Forth, then select/enter the designated UHF channel number but don't press the button straight away to scan/add services. Instead, wait and see what levels of strength and quality you get. Observe for a minute or so in order to see if there are any variations.