Mark, check that your remote doesn't need new batteries or that something hasn't been spilt on it - it could just be a coincidence that it happened during the switchover.
It was hinted that the signal would be stronger after the full switchover.
I'm not a great lover of one-for-all remotes - particularly for DVD recorders/players but, sometimes, 'needs must'.
If you have a good scout around the Internet, it seems to be getting easier to buy 'original' replacement remotes now - even for obsolete sets. Also, I managed to get a perfect used one for a discontinued DVD player off of e-bay.
I wonder how much extra would have to be added to the production cost of a TV, to make remote controls rechargable via a recharging dock on the TV?. I was surprised to find that the batteries on the remote for my DVD player, purchased last Christmas, were already down. Batteries are so expensive now, £3.99 for 4 AAA Duracell at Sainsburys, compared with the original purchase price of the equipment.
That's why, spurred on by my brother, I always keep a supply of AA and AAA rechargeables with varying powers. Of course, even they don't last forever but, after the initial layout including charger, they work out considerably cheaper and more convenient.
I have to admit that I'm still using the original batteries which came with my Sony remote more than two years ago, and the others get changed every couple of years, but each remote is different. You could go to Poundland, and buy 15 AAA batteries for a pound. The Sony/Kodak ones are perfectly fine for a remote.
All in One remotes were once described by Stephen Fry as 'instruments of the devil', or something along those lines. I've had at least two, and they seldom work very well, unless you want to spend a lot of money. However, if you have more than one bit of equipment from the same manufacturer, its likely that you can use the same remote for all of them, at least for the simple functions (rewind, fast forward, etc). Panasonic uses 'Vieira-Link', Sony uses 'Bravia Sync', and most other brands use much the same kind of system. Some of them will allow you to set up control over other manufacturers equipment as well. Check out the manual for the TV, etc - often the system has a lot more built into into it than most people use or even realise is there.
If you do want an original remote, there are plenty of websites which can get one for you, or if you are really pushed, try Ebay, as Dame Starry said. The remote for my now pretty old Eltax DVD was on the way out, and the only place I found one was Ebay. The prices was pretty good as well.
Some TV's are very slow about changing channels (one lady even came back into the shop to check hers wasn't on the blink - no, it was just slow). Of course make sure that the sensor on the TV has been cleaned (furniture polish is sometimes put on TV's, which bad, and it forms a layer over the sensor), as well as the remotes. You can always give them a good clean as well. A bit of rubbing alcohol (or even vodka!) will shift a lot of crud, and a cotton bud and cocktail stick will get muck out of the little crevices.
My brother would be appalled at using vodka to clean his remote/sensor - he'd say what a waste that was!
Remotes don't need expensive batteries - in fact batteries that have stopped working in some things, will very often last for quite a while in a remote or a clock.
It's also worth trying rechargeable batteries in a remote. I've had 'remotes' where they don't work but some where they do and last for a reasonable time.
So if you get bizarre results, check the power supply. A volt meter said it was giving 12V, as it should, but I bet there was either a lot of 50 Hz hum getting through or else it wasn't supplying the full 12V when it was under load, only with a high resistance voltmeter.
God knows why the OnDigital box was also playing up, unless it was so old that it wasn't compatible with modern DTTV signals.
To get no indication of signal strength or quality suggests something sinister and fundamental: you'd imagine that the box's signal strength display is probably going to be strength of the UHF signal before any attempt has been made to decode it.
I've just had a thought. I wonder if even when I was testing the OnDigital box, the duff PSU for the other box was still plugged into the mains. Could the PSU have been chucking out mains-borne or RF interference that was upsetting the OnDigital box?
It's a shame that Windows Media Centre doesn't have a signal strength meter. I've had occasional problems where recordings on one particular multiplex have terminated with "no signal" or else have been so severely corrupted that they are unwatchable, whereas most of the time the recordings are flawless. I'd be interested to be able to compare signal strength for the two cases. It's the lowest-power mux from the transmitter, so I suppose changes in in signal propagation might be the cause.