I'v posted a question and had a response.

Now I want to reply to the Response, but I can't work out how to do this!

You can simply type in your respond in the "Quick Reply" pane and your reply will be attached to the last post in the thread. Alternatively, you can click the "Reply" command in the header of the respond to reply to the response.

Thank you.

There must have been something wrong earlier today since I couldn't see the Quick Reply pane (as I can now!), and none of the commands in the header was available to me!

Can you see the commands Edit / Reply / Quote / Quick Reply / Print This post / Bookmark Post / etc ... on the right of the header pane of each post now?

If you can't which browser / version are you using?

You were perhaps not logged in during that session when there was no Post pane. (Only logged in members can post).
It is indeed preferable not to use QuickPost, to reply to the specific post you're targeting so that the member in question receives a notification email of your reply.

Cheers.