@Claire158.....Hi Claire, there are always ways to handle a booking problem without you having to cancel! If a guest wants to alter dates that they have booked, let them do it without involving you. Your listing is available or it is not, and if your guest wants to change dates, that is their issue, not yours! Work with them, but not FOR them!!!
As to the plumbing problem, you could first message the guest and explain why the hosting cannot take place and ask them to cancel their booking and choose another listing. Or get onto Airbnb help desk and ask them to relocate your guest due to extenuating circumstances.
Claire, you must exhaust all avenues before you cancel a confirmed booking. The booking system is automated and does not take into account that you felt you were helping or doing the right thing.....it just recognises that you cancelled a confirmed booking.
You are right, this will not look good in the short term, and it may have an effect on your bookings....host cancellations are looked on as 'the kiss of death' where guests are concerned because, they don't want it to happen to them, and they will regard a cancellation or two as a sign of unreliability as far as the host is concerned.
As other contributors have suggested you should either phone Airbnb in England on +44 203 318 1111 or else through the Airbnb Facebook page and plead your case and see if Airbnb will remove these postings and release you from future penalty.
Claire I see you have many successful hostings and your reviews are wonderful but before you do more damage to the wonderful work you have done it would pay to sit down with a cup of coffee for an hour or so and familiarise yourself with Airbnb's rules of hosting so that you can avoid this sort of thing happening in the future. On the forum home page just type 'Help' into the search bar and a lot of options will appear that will give you the knowledge you really need to know before getting too involved with hosting.
Good luck and I hope it all works out for you...cheers.....Rob