Someone else mentioned it, but if you asked me to think about it, I'd have got it

Not 18 months, but about 30. And the end of 2006 was the last thing that would have sparked
caution - after a run of over 100 games without the £250k coming to the table, it was sold at eight-box in two games in a couple of weeks by emotional players (Ingerlise and Donna). The result was Noel getting more and more agitated about it never being won, then Laura (having been a bit rash for her utility curve in turning down £19,999 on a one-box game at five-box) was given a free pass with the final offer, and the players just got carried away for months after that, Noel doing rather more to encourage them than to stop them. However, the offers were so generous in those games that even those players ended up dealing eventually most of the time if they had very strong boards - only Bunney came close to gambling for the £250k, and I think he wasn't sure enough about the swap to commit to a No Deal!
Then in May 2007 a player had a crash, took it horrendously, and players were cautious for weeks afterwards. And then we got to episodes taped after the fall of Northern Rock - which as far as public perception goes is the start of the recession, even though GDP kept rising until late 2008.