Roger Burton - in case you're like me and on a phone exclusively (or even not exclusively

, when you first click on the topic, if you just barely scroll down, you'll see two green arrows appear in the lower-right corner; touch one and it goes all the way to the top, touch the other and it goes all the way to the bottom.
That'll get you down to the bottom in a flash, and then you'll see the little posting form at the bottom. It doesn't matter if you're on the first or last page of a topic. It'll be there regardless, and you can post away.
Also, it's a little disorienting at first, but if you do that from the first page, it'll initially look as if you've posted to the first page. You'll see all the responses in the first page, and yours tacked on the bottom, but it's really at the bottom of the last page. Sounds confusing, but it's not.
Lastly, it you prefer to go to the last page of the thread, you can do that by hitting the tiny little "fast-forward" arrows near the top, by the Next and Prev buttons. That'll take you to the last page of the topic, then you can scroll to the bottom of that page, and there's the posting form, ready to go.
I'd do screenshots, but I'm cooking breakfast and typing with one thumb.