Inadvertently, this turned into a journey of testing several out there on the market
So, I decided I would share my findings 
and bore the rest of you to jobby in the process 
First up;
Streetwise 12v trickle charger for £21.99
http://www.eurocarparts.com/ecp/p/car-a ... &0&cc5_145
http://www.streetwizeaccessories.com/Ca ... rger-SWTBC
Ok, not actually a smart charger, but it is a automatoc one that you can leave plugged in (auto on/off). Inexpensive, can pick it up just round the corner (eurocarparts)... Tiny, cheap looking thing so was very dubious about it. In particular, the first week I left it switched on at the barn I was shitting myself all week that I'd get a call from the landlord telling me that he was very sorry, but, the barn had burnt down and all my cars were gone...
It's not massively happy charging a completely flat, big battery, but it does it and then cuts in/out fine. It's now been in use for 6 months with no problems. It does get a bit warm though and no IP rating, so bear that in mind when you place it. However, it's a very very basic spec in comparison to lots of other stuff out there and not actually that cheap either, once you really start shopping around.
Second, I bought the CTEK MXS3.8 I paid £47.99
http://www.smarterchargers.co.uk/12-vol ... duct_id=52
These are the absolute bollocks. Full stop. Awesome bit of kit, proper automatic charger with 8 step charging modes and IP65 rated do can leave it plugged in outside in all weathers. I would have loads of these if I could afford to, but... They are more pricey. You can get them cheaper than that link I put above by shopping around on ebay (and you may be able to pick one up cheaper now than what I paid) but they're still far from peanuts.
Next up I tried this;
Defender 4A 12v Automatic Electronic Trickle Charger for £20.99
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... 4354645011
You can get this charger branded as Maypole as well (and probably others). IP65 rated so can leave it outside, digital display and was very cheap. Absolute bargain, yes?
Well, no... It was jobby! Tried it on 2 ludes where the battery was flat. First one it refused to detect it as a 12v battery (would only set itself to 6v bike battery and wouldn't let me manually change it) and never switched into 12v mode. The other one, it detected as 12v but after 2 minutes, it said the battery was drokked. Tried it several times, tried switching between modes and same result throughout. So I got my ancient 'thick' 4amp trickle charger out, plugged that in and both batteries charged up fine.
So, this one is probably ok as an automatic trickle charger for good batteries, but... It's marketed as a smart charger that can recover/de-sulpherise old batteries and... Clearly, it can't handle a battery than is in anything other than tip top condition, so it's useless for what it's marketed as.
Maybe I got unlucky and there was a problem with mine, but I'm dubious and sent it back for a refund - the seller was great about all that tbh. Well impressed with them.
So next up was this one;
Excelvan 5Amp Battery Charger Maintainer for £26.79
http://www.ebay.co.uk/itm/112230228815? ... EBIDX%3AIT
IP65 rated*, good price, UK distributor (no waiting for slow boat from China) blatent copy of the CTEK MXS3.8.
Tried it out today and so far, seems great, just like the CTEK. It's recovered the battery that the Defender failed to (it had gone flat again since the last charge).
So, there you have it.
(
Anyone still awake? )*note that I had a problem with one of my excelvan chargers that means this IP65 rating isn't true.
It was left out in the rain, sat on the drive completely exposed. Next day it was still on but the cover over the led's obviously had condensation inside and it was flicking between modes.
Switching it off and carrying it inside showed that when tipped up a particular way, water drained out of the corners, through the seal in between the two halves of the case. Took it apart and there was a small amount of water inside. Nothing wrong with the seals etc. Contacted the seller and they were great - offered me to return the item for full refund, or keep it and get it repaired and have £6 refunded. I dried it out, cleaned it up, reassembled and it seems to work OK, so accepted a £6 refund. I've concluded this might either be a one off fault with mine, or is a design fault with the shape of the case, which means the 2 halves of the case at the corners that stick out and have the through holes for bolting it to something, aren't tightly enough clamped together due to being too far away from the screws that fix the 2 halves together. Would be fine with an additional through bolt through those holes, or just ignore the IP65 rating and consider it as something like IP62 or 63 instead (can leave outside but keep it inside the engine bay away from direct rainfall). It's still a great charger and highly recommended though, despite all that.
