Uk Postage April 2nd 2013

Okay, have a customer who sells socks:
Socks are of varying thickness and length.
Post office has now changed their pricing to a combination of weight and size, this is a nightmare for automated shipping!
I could work out how many of one sock would fit in to a small packet, but if the customer had one of the largest thicker socks then the weight is under 1 kilo but the dimensions are now larger than a small packet!!

www.postal-calculator.co.uk/postage-rates-calculator.asp

Example:
My customer can get 3 pairs of one type of sock in to a small packet for (weighs less than 1 kilo) £3.00 but if they then change one pair to the largest sock which still weighs in at less than 1 kilo the packet size then goes up to a medium size packet of £5.60 for up to 1 kilo!!

Anybody know a way of calculating the new postal tariff for the UK Post Office?

Many Thanks

Ian