There's loads of them now, they've been rolled out in lots of places, I think when Rugby was modernised was first.
For clarity I assume you're referring to banner repeaters (my first thought was old style signal heads with LED light engines replacing the traditional signal lamps!).
They're the new standard - most resignalling schemes now incorporate them. Probably been around for 3 years or so now, at a guess.
Longer than that. The banners on the Chilterns were fitted with LED type heads during Evergreen 3, and that was back in 2011. Dorman (one of the major manufacturers of signalling equipment) announced they had a secured a contract to replace filament banners with LED types back in 2009 and had been replacing filament position light signals with LED types as long ago as 2004. I guess with trials you might find there are locations with LED banners in place which are at least a decade old.
If you mean Streatham Common on the Victoria - Brighton main line, these new signals were brought into use a few weeks ago (the early May bank holiday weekend, or the previous weekend, IIRC).
Yes, WH75BR, one of the new 'Tri-state' banner repeaters, was installed on the Down Fast, repeating the signal at the North end of St Albans City Station platform 4, in July 2013. This was part of the preparations for increasing the line speed last year.They have been up towards Pancras <> Bedford for quite some time.
Definitely started to appear on the WCML as part of the upgrade works. IIRC there are examples visible from platforms at Lichfield Trent Valley (Down) and Wigan North Western (Up), the former would likely have appeared at the same time as the aforementioned example at Norton Bridge, the latter as part of the slightly later Wigan area scheme. As usual their use is dictated by a combination of sighting distance and linespeed.
In what way is conventional signalling increasing in complexity or cost? A lot of the new LED signals (not just banner repeaters) are like for like swaps. For example, the resignalling of the Gresty Lane area of Crewe has seen minor signal relocations, but on the whole like for like swaps. Traditional filament bulb signals have been swapped out for LED signals and Gresty lane 'box has also gone; control moving to a new panel in Crewe power box. There is a saving on the upkeep of a knackered old structure straight away.
Given that LEDs have a much longer service life than traditional filament bulbs,it seems odd to think that replacing something that will last ten times longer is a cost increase.
Can't say I've seen many banners 'mushrooming' on the routes I sign, but as they are an aid to the aspect of the signal I can't see around the corner, they are a welcome addition.