Thank you all for your responses. Since my original post I have discovered the TrainSplit website (is that your creation, SickyNicky?) and I've used it to find a split for the two journeys I like best...
Outbound: 10:17 - 14:45 (4 Hours, 28 Minutes)
From Gillingham (Dorset) To Camborne
Changing At Exeter St Davids
Return: 16:21 - 20:50 (4 Hours, 29 Minutes)
From Camborne To Gillingham (Dorset)
Changing At Exeter St Davids
GILLINGHAM (DORSET) to NEWTON ABBOT
OFF-PEAK R Return £24.50
(outbound: VALID AS ADVERTISED )
(return: VALID AS ADVERTISED )
Route: Valid via any permitted route.
Restriction code WG - please read this important ticket information
NEWTON ABBOT to PLYMOUTH
OFF-PEAK R Return £12.10
(outbound: VALID AS ADVERTISED )
(return: VALID AS ADVERTISED )
Route: Valid via any permitted route.
Restriction code WG - please read this important ticket information
PLYMOUTH to CAMBORNE
OFF-PEAK R Return £19.20
(outbound: VALID AS ADVERTISED )
(return: VALID AS ADVERTISED )
Route: Valid via any permitted route.
Restriction code WG - please read this important ticket information
Total Fare £55.80
If I had picked two journeys that both called at Totnes then I could have got it down by another couple of quid (as someone said above), and it looks like I could knock a further tenner off by switching onto an advance fare at Newton Abbot, but I've decided that I don't want to tempt fate with this particular trip!
Thanks again for all the suggestions (what a shame I don't have a railcard for the FOSW), and since I won't be purchasing until September now I'll remain open to hearing anything better that anyone finds before then!