I have learnt, elsewhere, that the answer lies in:
Section 53, Schedule 10, Venture Capital Trusts, pages 361 to 362
2 (1) After section 264 of ITA 2007 insert—
“264A Restricting relief where there is a linked sale
(1) This section applies where—
(a) an individual subscribes for shares (“the relevant shares”) in a VCT (“the VCT”), and
(b) there is at least one linked sale of other shares by the individual.
(2) For the purposes of this Part, the amount the individual subscribes for the shares is to be treated as reduced (but not below nil) by the total consideration given for the linked sales of other shares. This is subject to subsection (3).
(3) If a sale is linked in relation to more than one subscription for shares—
(a) the consideration for it is to be applied to reduce subscriptions under subsection (2) in the order in which the subscriptions are made, and
(b) accordingly, to the extent that any consideration has been used to reduce an earlier subscription, it is not available to reduce a later one.
(4) A sale of shares (“the sold shares”) is “linked” if conditions A and B are met.
(5) Condition A is that the sold shares are in—
(a) the VCT, or
(b) a company which is (or later becomes) a successor or predecessor of the VCT.
(6) Condition B is that—
(a) the individual subscribes for the relevant shares in circumstances where—
(i) the purchase of the sold shares from the individual was conditional upon the individual subscribing for shares in the VCT, or
(ii) the individual’s subscription for shares in the VCT was conditional upon that purchase, or
(b) the subscription for the relevant shares and the sale of the sold shares are within 6 months of each other (irrespective of which came first).
(7) A company (“company X”) is a “successor or predecessor of the VCT” if— etc etc.
So, buying first and selling old shares afterwards doesn't get around this nuisance and I'll have to follow the alternative approach of either (a) selling 'old' holdings at the start of a tax year to have the greatest chance of a six month gap before any new issue or (b) buying Northern and selling Baronsmead, or whatever.