SO, playing around with different insulation types... there's expanded polystyrene, the white stuff used for packaging, also known as beadboard because its made up of, and breaks up into, little beads... (U value = 0.35/100mm, .7/50mm) and Polyurethane sheet, the more rigid and sometimes structural (U = 0.21/100mm, .42/50mm)
100mm Polystyrene works out about £5/sqm on eBay, Polyurethane (rigid insulation board) £12/sq m
50mm Polystyrene works out about £2.6/sqm on eBay, Polyurethane (rigid insulation board) £6.5/sq m


So which to use?

In my workshop, total wall area = 42sq m. (50mm insulation), roof area = 24sq m (100mm insulation)

So Polystyrene, heat loss = (42 * .7 + 24 * .35) * 20 = 756w, Polyurethane, heatloss = (42 * .42 + 24 * .21) * 20 = 453w

Cost of electricity = 12p per Kw/hr
therfore cost per hour difference = (.756 - .453) * 0.12 = 3.6p/hr

Cost of insulation, polystyrene 2.6 * 42 + 5 * 24 = £229, polyurethane 6.5 * 42 + 12 * 24 = £561

Polyurethane pays for itself after (561 - 229)/0.036 hours = 9222 hours, or in reality, slightly over 5 years assuming only gets that cold 20% of the time (and I'm not in there 24 x 7!)

Since I'm working to a small and diminishing budget, polystyrene it is for the workshop....

Obviously for a new house build polyurethane is the better choice... (its more stable, U value doesn't change with time, and the additional cost is a small proportion of the total build cost)