Dave,
Shemy could be anticipating a higher level of exits.
As for Typhoon numbers increasing that is true. However if the Typhoon number increase is matched by the Jaguar and F3 runout then some of the increased Typhoon seats will already be filled by those pilots who remain.
The 120 figure already mentioned is the annual quota for 07/08. This will be re-assessed later this year to match the actual in-to-productive service numbers from 04/05 and the expected retirements in say 09/10.
A shortage today cannot be addressed by a rapid increase in recruitment as that shortage might evaporate of its own accord by 2010. For instance there may be a smaller number needed in 3 years time or more pilots may remain in-service.