Landlord trying to charge for communal maintenance.

My partner and I moved into a flat last December. It's in a converted house with 6 other completely separate units. Yesterday we received an email from the landlord that...

"Once again it has come to our attention that someone has been putting wipes (and other unsuitable matter) down the Toilet/s and we have yet again had to call out a drain company to clear the properties main communal drain.

 On this occasion the landlord has had to stand the cost to clear this blockage, however, should this occur again, unless the tenant responsible comes forward, we will have no choice other than to spread the drain company invoice cost equally between all the Apartments/tenancies, therefore it is in everyone’s interest to cooperate and ensure this is avoided going forward."

Can they really charge us for a communal cleaning bill if they can't prove we are the problem? We definitely aren't by the way. Also, this is the first time we've received contact from the landlord about this issue. If the time that they had to get the drains unblocked was before we moved in then surely it must be someone who has lived here longer than us, no?

From our rental agreement...
"8.2.11
Not to flush any sanitary waste, nappies or wet wipes down the toilets and to be responsible for
unblocking or clearing stoppages in any sink, basin, toilet, septic tank or waste pipe which serve such
fixtures if they become blocked with the Tenant’s waste, or as a result of the actions or inactions of the
Tenant (or his invited visitors or guests) in breach of obligations under this agreement.
8.2.12
To clear or pay for the clearance of any blockage or over-flow when any occur in any of the drains,
gutters, down-pipes, sinks, toilets or waste pipes which serve the Property, if the blockage is caused by
the negligence of, or the misuse by the Tenant, his family or any visitors."

Should we acknowledge the email now and ask when the previous maintenance happened and begin to fight our case now?
or wait until the issue happens again if it does.

thanks for any and all help and advice