We had a similar problem in L'Aberach last year. Coming home we used bus to Brest and when we arrived there we found there were no trains of buses to Roscoff (despite it being major city to the nearest cross channel port) after 6 in the evening. We then used a taxi which cost about £70, but divided by four for an hour or more's journey was actually not a bad deal. (Was probably cheaper than the train fares for the four of us.)
On the way back we just got a taxi from the ferry terminal in Roscoff. It was about £50 and once again between three of us door to door, I though it was a bargain.
There is a rail link to Roscoff, but I must admit I have never seen a trian on it. The Paris train to Morlaix is met with a bus for the final few miles, not sure if this is the same for the "up" train from Brest. Sorry, either way this will not help you Mick. Looks like the taxi option. Brian.
Many thanks to all who have offered information here. As I suspected there are no easy options for this journey. Maybe some enterprising type should run a weekly minibus from Blighty to the Brittany marinas.
We parked our 30 tonnes of honduras mahogony there a few years ago on a hammerhead which was less than half the length of the boat. When the tide bit we lifted most of the pontoon clear of the water and put a pretty little bend in the whole length of the walkway.