So far as I understand it, the round trip option is from one point.

I imagine you could ask CT to plot a round trip from your start point and manually add a via point which then gets dragged to the second place you want to visit. 

If it's acting up in terms of distance then maybe try saving the first suggestion, reopening that route and then add the extra via point? 

Thanks for the quick reply 'Hobbesy' (if I may...) - I'll give that a whirl

Beware! Hobbes is a tiger with big teeth and currently on the loose in locations unknown! :-)

PS. Weirdly it's just done what I expected it to do in the first place... came up with a round trip after I entered 2 points. Must have been having an off day.

It depends on the location! Generally it can find a round-trip for longer routes, but will sometimes struggle on shorter routes. Essentially if there’s more than one decent A-B route without too big a mileage discrepancy, it’ll find something.

It does also occasionally struggle in areas with comparatively few roads – e.g. long-distance rides across Scotland. (That’s why it’ll find a round-trip from Land’s End to Edinburgh, for example, but not all the way to John O’Groats.)

Thanks Richard – the round-trip it found (Between Criccieth and Aberderon) looked good on the computer, but didn’t work on the Garmin which seemed to think its work was done at the halfway point. 
Thankfully I have the app which was happy to give me the whole route. Great riding!