Simple tax?

I should be grateful if readers could consider the following share history :

50,000 shares
bought 8.11.99 £6,310
50,000 shares
sold 11.11.99 £5,600
50,000 shares
bought 1.12.99 £11,835
20,000 shares
sold 6.12.99 £6,675
5,555 shares acquired
under a 1:9 rights issue
9.12.99 cost £444 (by virtue
of 50,000 held on 1.12.99)

Under the share identification rules, the disposal on 11 November 1999 is matched with the acquisition on 1 December 1999 (being an acquisition within 30 days of a disposal), giving a net loss of £6,235. However, the rights issue is treated as being acquired on the same date as the shares from which it arose, in this case the acquisition on 1 December 1999.
As the 1 December 1999 acquisition has already been matched when the rights issue takes effect, should the new 5,555 shares be treated as a separate asset, as the original shares to which they would normally be added are theoretically no longer in existence? Therefore, it would appear that the subsequent disposal on 6 December 1999 should first be matched with the rights issue (5,555 shares), with the balance (14,445) being taken from the shares bought on 8 November 1999.
Or should the disposal on 11 November 1999 be matched with 50,000 of a combined 55,555 treated as purchased on 1 December 1999? The subsequent disposal on 6 December 1999 is then matched with the remaining 5,555 with the balance coming from the shares bought on 8 November 1999.
Is there not a danger of re-writing history if the latter is correct? If the rights issue was made on, say, 6 June 2000, based on holdings on, say, 2 December 1999, the 1999-2000 tax return would have to be amended as the amount and cost of shares acquired on 1 December 1999 will have now changed.
I should be grateful for advice on the correct treatment.
(Query T15,725) – Lighten our Darkness.