Blocked drain in apt - plumber says issue is in apt. Who pays, tenant or landlord?

This is more a general question as opposed to looking for the legal position (as in our contract with the tenants we had a point stating blocked drains were the responsibility of the tenant).

Background:
We have good tenants (we have them a couple of years now) and are happy with them
The apartment has a blocked drain (toilet and sink are very very slow to clear)
After inspection (paid for by us - landlord), the plumber said the issue was within the apartment drain, not in any shared area in the apartment block (I'm not 100% clear how he came to this conclusion but am taking his word for it as there is no reason to doubt him)

Question is, would you expect the tenants to pay to resolve this or would you (as landlord) pay it? As they are good tenants I don't have any huge objections to paying it as the tenants assure us that they only put toilet roll down the toilet but am just interested if others have had this issue and how did they deal with it.

Good tenants as you say, I would pay for it if it comes to that.

Did the plumber put a camera down the pipe..?, when the plumber was there were attempts made to clear it...?.. have you exhausted drain cleaner products..?

Palerider said:

Did the plumber put a camera down the pipe..?

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Not yet, that was his suggestion as a next step

Palerider said:

when the plumber was there were attempts made to clear it...?

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Yes, he tried to "flush the system" - am not completely familiar with what that means in layman's terms!

Palerider said:

.. have you exhausted drain cleaner products..?

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Yes the tenants have tried a number of different products to clear the drain (including caustic soda).

What has blocked it? Is it something that tenants are flushing through such as nappies, wipes or fat?

We don't yet know what has blocked it (hence the suggestion to put a camera down next). All we know at this stage is the tenants say they only put toilet roll down there and the plumber says the blockage is in the apartment pipes.

Under normal circumstances a landlord is responsible for the drains in a property. However, if it can be proved that a blockage was caused by the tenant then the tenants pay for the unblocking.

If the blockage was caused, for example, a nappy and the tenant does not have a baby then it could be difficult to prove that s/he caused the blockage - it may have been a previous tenant so landlord is liable for the unblocking and probably too late to claim from the previous tenant.