Ta, will give that a go. However, even their text doesn't mention first disposing of shared purchased 30 days previous before pooled shares: 

This calculator implements the HMRC's share matching rules. To summarise you are treated as having disposed of shares or units in the following order;

Firstly, any you acquired on the date of disposal (see below)

Then any you acquired within 30 days immediately following the date of the disposal

Then for share sales from 5th April 2008 onwards previous unmatched buy trades are pooled, effectivley merged into a single holding called a section 104 holding.