Can they rely on their portal to provide all the information that should be in the LBC?
I think some people have complained to the SRA about being disadvantaged by this. In my view they shouldn't be insisting on the use of proprietary software to access documents they're required to provide as part of the pre-action protocol. Pragmatically, if you think you may need to rely on any of the documents provided on the portal to assist with your defence of the claim, you may choose to use it.
Are they obliged to provide details of the contract between the land owner and PCM?
You could raise it as an issue in your defence (ie put them to proof they have such a contract), although it is a rather speculative line of defence, so I wouldn't lead with it.
Does the latest communication need a further reply, or do we await court action?
In my view, no. You've set out your position, they've set out their position. There's little to be gained from an endless ping pong of letters.