I have long believed that in 1993 Paddy Ashdown made a Faustian pact in which he was allowed to choose any constituency in the UK for a by election. Clearly choosing Richmond or Hazel Grove - long on the Liberal/LibDem hit list - would have been insufficient, so he chose Newbury. A 12,000 Conservative majority was, in the circumstances of the day, easy to take out, yet it was a seat that had been Conservative since it was first created, and is again now.
What if David Cameron was offered such a deal? What seat would he choose? It is tempting to take advantage of the LibDems current disarray and choose, say, Hereford. But they have a very effective - and very dirty - campaigning machine. It is often difficult to predict how things will go. Safer to choose a Labour seat. There is no point choosing Dorset South, or Dover. These are the sort of seats the Conservatives win when doing badly, but not disastrously.
A better choice would be a seat the Party has not held since the 1980s. Birmingham Northfield, for example. This would have the added bonus of gaining back representation in the UK's second city, ahead of council elections there in May. Perhaps, symbolically, a seat in Scotland or Wales would matter more. Yns Mon springs to mind. The Conservative Party was fourth there last year, behind Labour, Plaid Cymru and an independent conservative. The combined conservative vote was 9,000, behind 11,000 for Plaid and 13,000 for Labour. Third place, at best, but still very winnable.