Not honouring initial price offered in email

Hi - I enquired about our David Lloyd in August that is opening up and they offered me 160 founding member rate. No expiry date is listed in the email although they state the "prices will go up". I went overseas for three weeks and then upon return they have quoted 200. I joined as I didn't want to be subject to any further price increases but can I hold them to the 160? It seems unfair as if I knew it was expiring while I was away I would have joined up.