No prob, Tyson.
Don't bother with the Maplin supplies; they're ok for what they are, but they're old designs and have low outputs for their large size. Even their biggest one is only rated to 7A constant, which isn't enough if you're going to be doing any serious charging. They're also kinda expensive. Pro-Peak do a fantastic 20A 13.8V supply for about £40 (Modelsport and other places sell them), and a twin-output version for a bit more. They also have a much more compact 8.5A supply for about £25.
The LRP/Nosram supplies are nice and small, but overpriced IMO.
Alternatively, if you're good with electronics, then you could always try converting any spare computer PSUs you have lying around as CFR said. Be careful though, and do your homework before you do it; it's potentially dangerous if you're careless.
Do you have a link to the 15A PSU you're talking about? 15A should be plenty of capacity, as long as it can out put clean, stable voltage when under load.