diy
the screwfix looks ok - the trouble is - you aren't familiar with electickery
my advice would be find someone who is
burning the house down is a real option here - these transformer/load run at such high power/temperatures that you don't get a second chance (7000mA= 7 amps - you don't mess with 7 amps)
if the maplin is faint
I suspect that you are running the lights in a daisy chain (series) ------x-----x------x-----x------
rather than parallel ====x====x====x====x
which is why ....
series shares the voltage - so 4 at 12v = 3 v to each bulb
so the christmas lights use 20 12v bulbs in series - which plug directly into the mains (you can't beat sitting round a good fire at christmas!)
parallel maintains the voltage and so increases the power drawn (current) - so it works hot and bright