Griffo wrote:
I'm insulating an internal wall.

Why?!

By the way, I have had a lot of trouble trying to find insulation
board. Could someone recommend a good brand, depth and supplier.

Kingspan, Celotex for a start.

Depth depends entirely on how much insulation you need, ie what you're
trying to achieve.

A good supplier is:
http://www.secondsandco.co.uk/ - they sell literally 'seconds' quality
insulation a lot cheaper than you're likely to get it from a builder's
merchant.

David