VAT and receipt configuration

I've RMS set to use VAT, as opposed to Tax-exclusive, within SO Mgr->Configuration->Sales Tax. I have a client that requires receipts to show totals exclusive of VAT for a small percentage of his customers.

I was reporting the price as Entry.ExtendedBasePrice for each line with the ExtendedBasePrice / Quantity to provide an ex-VAT price, which appears to work ok to a point.....

Scenario:-

1) Create a workorder transaction of 3 lines = 3 items @ £39.99, 1 item @ 6.18 and 2 items @ 0.99. Ex-VAT of £34.03, £5.26 and £0.84 respectively. 2) Pickup 2 of item 1 and 1 of item 3, leaving 1 of each item remaining on the work order. 3) Pickup with remaining items.

This produces 3 receipts, one for the inital order and 2 invoices for the pickups.

*The Ex-VAT prices on the last receipt are wrong* £38.00, £5.26 and £0.92 respectively.

This issue occurs using Entry.ExtendedBasePrice and (Entry.ExtendedBasePrice / Entry.Quantity) only, no other fancy formulae.

The fact seems to be that the internal values in RMS seem to be plain wrong for the 3rd transaction, unless I'm missing something really daft. I could start calculating all tax manually myself, but this to me is reinventing the wheel.

Can someone point me to my mistake? Or have I fell down a VAT blackhole?