Thanks Simon and John!
Simon wrote: "I've just realised that even though they played the Rochdale coconut dance in the town of Bacup, The Bacup, which they play with coconuts, is not the same tune!"
Ah that explains it then! I hadn't noticed a similarity upon first impressions, but I figured it was probably there and I'd just missed it somehow.
Thanks for that link. It has both tunes (Rochdale Coconut Dance on page 143, and Tip Top Polka on page 176), so I put them side-by-side in two different windows (on my computer screen) to compare them. Yep you're right, two different tunes.