Hosts not reacting to guest requests in a timely professional manner are penalized by having the dates blocked.
You'll need to contact Airbnb if potential guest is still interested to see if it's possible to get the dates unblockedf and send them a so-called special offer (just repeat your price and send to reopen the request).
Is this a new regulation that has come in from BnB - I wasn't aware they would block your calendar if you didn't reply within 24hours. I thought they only block your calendar if you cancelled a confirmed booking.
Can you link me to the section on their website where they talk about this?
I replied to a request, detailing the reasons I wanted to delay accepting the request ( 5 months in advance!) but airbnb have noted that i failed to reply. Does this mean I am penalised for not accepting or declining?
You didn't take action in accepting or declining is why. Now Airbnb will have blocked the dates. You really have to make a decision within the 24h.
If you can't decide, it's better in the long run to decline and tell the guests you are declining since you can't decide yet, but are willing to take up the case later and they are welcome to resubmit at such and such a time.