I'm really doing poorly with my putting at the moment and I'm thinking of getting a training aid of some kind.
Anyone got any recommendations? These chaps: www.meritgolf.co.uk come up on certain searches for that type of thing. Anyone purchased stuff from them? Eager to sort my putting out in time for summer competitions but don't want to waste my time or money on something that won't help ...

Best two putting aids I have used are

A penny.

Putting to roll over a penny rather than to put the ball in a hole, narrows the target and makes you concentrate on a specific part of the hole rather than just the hole itself.

Carbite Sergio Garcia putter ball

Shows up any strike which is offline instantly.
http://www.direct-golf.co.uk/product/product.asp?product_ID=379

Best putting training aid, if it can be called that, is the book called "putting out of your mind" by Bob Rotella.

Read Dr Bob!

Cheers guys ('',)

Seriously read Dr Bob Rotella's 'Putting Out of Your Mind' and then practice until you groove your own putting stroke.

The 1st step is to ingrain your own repeatable putting stroke so you know exactly what line the ball will roll when you putt it.

I used to be a truly attrocious putter, but I read Dr Bob and then practiced daily, (putting into a golf-hole sized putting aid until I reached where I am now, (a pretty decent putter who can't remember the last time he three putted),
There really are no shortcuts with putting, until you know you're stroking the ball along the line you think you are, you will always struggle on the greens.