Looking at the AMD Athlon X4 760K, but wondering if I spend another £20/$35 am I going to get anything noticeable better?
Well, about all you can get that is better is the FX4300, which isn't exactly much better. It will be within the margin of a few percentage points difference and so probably ins't worth spending the extra money on.
What about the A10-5700 or 6700?
(limited to FM2+ due to the mini IYX form factor)
They won't be any better either. The Athlon X4 760K is essentially an A10 5800k without the the GPU. So, unless you need the GPU on the APU, the Athlon is still better.
Thank you!