"So, if 6 fast-jet combat squadrons are not enough then how many will the RAF need by 2020 to be credible and effective?"
Credible to do WHAT?
All depends on the mission - if all we're going to do is intercept the odd LCA who has forgotten to change frequency or deter the ravening hordes of Wee Ecks Army then 6 squadrons is too much
If we expect Mr Putin to decide to "protect" Russians living in Mayfair then we'd need 60 squadrons