roger -first thing is make 100% sure that your "paperwork" is in order - tenants are PRTB registered, rentbooks, contracts etc. I regret that even if you are in the right you'll lose much credibility in any disupte if you aren't absolutely in order.
second thing is - choose one of the tenants that you wish to evict. Trying to evict both may look like your a difficult landlord -plus if they're both against you you'll get even more stress.
Third- did -or will- any of the plumbers give a report stating they found no cause for the leak?
Fourth - even if you are quite sure that the tenant caused the leak , how is the tenants behaving in other aspects ? It may be gratifying to get rid of the tenant but if there are no other faults or problems then ,frankly, why put yourself thru the stress?
i'd probably go for the tenant who is always late and is causing continual problems.
but ,sadly, in this country nearly evrything is on the side of the tenant. And you can't do anything unless you put a warning letter i nw riting 9except in dire emergenceies).
Ask again if you need more details..