I have always found Maplins to be pricy. I'm sure you will get the same spec for less elsewhere - or a better spec for the same money.
Building your own is very satisfying but unlikely to work out cheaper, unless you are utilising 2nd hand parts / spare parts / ebay finds etc.
I've built 3 of the 4 current home PCs (all with same generation AMD CPUs) from scratch, based on a combination of the above parts with new where necessary. All happily running Win 7 with rock-solid performance. Now considering SSDs to replace boot drives.
SD