Darren,
It's simple supply & demand.
If you look at the links to N&B that you posted, the new 5.2Ah battery is £91.74 list, but with a 28% discount is sold at £66.00.
The new slimline 2.6Ah battery is £81.54 list price (£10.21 cheaper than the 5.2Ah). The discount offered however is lower at 11%, therefore the selling price is £72.75.
Festool dealers buy from Festool at different prices depending on quantity. On products with a higher demand (like the 5.2Ah battery) they can buy more of them, so get them at a lower price allowing them to offer more discount. Products which are less popular (like the 2.6Ah battery) they buy in smaller quantities, so they don't get as good a price. Hence, they can offer less discount on those.