Jim and Sarah
I am not sure whether you are wanting specific information about getting back to Bayonne with a tandem, as in the original post, or more general info about getting back from Santiago with a tandem. We did the Camino by tandem earlier this year so can provide some general tandem related information. (we are also both 55 years old!)
We cycled directly from home in the UK, ferry to Cherbourg and across France, meeting up with the 'standard' route at St Jean Pied de Port. To get to St Jean with the tandem, but not cycling, you can probably use the French trains to Bayonne. I understand bikes are only allowed on TVGs if boxed and within certain size limits but you can probably use the slower trains. An alternative would be the 'bike bus' (European Bike Express) which is a coach service for cyclists and their bikes from the UK to France and they have a drop off at Bayonne and Lourdes.
Our intention had been to cycle to Santiago and return by ferry from Santander with the tandem, which we had booked before leaving. If we had time, and the strength/inclination, we could cycle back to Santander or get back by some other method. We had found from various sources that it was difficult, but possible, to take bikes on trains in Spain but it was almost certainly not possible to take a tandem and therefore this was ruled out. The main option was therefore to do a one way car hire from Santiago to Santander with the tandem in the back.
However, on arriving in Santiago we realised there was another option, of sending the bike home and returning without it. All the companies gave rates for solo bikes, about 90e to the UK, but no information on tandems. The first company we approached quoted double the solo rate, which I thought was a bit cheeky. The second quoted well over 200e. We next got in touch by e-mail with someone we were approached by at the pilgrim office and after some delay got a quote of the solo rate plus 20e, which seemed very reasonable. The web site is
www.sendyourbike.com, which has other contact information on, and we dealt with Ismael Garcia who spoke good English. The company running it is Prodigal Transport and although I am not sure they had actually dealt with a tandem before they were very helpful and it was collected at our accommodation in Santiago and arrived back in the UK well boxed and handled by UPS. Another alternative would be to fly back and check the tandem in as luggage. This very much depends on the airline involved and would need careful packing/boxing in Santiago. We have done this a number of times in the past but also had the tandem damaged twice so now avoid it if possible.
Hope you get things sorted and I am sure you will have a great time.
Buen Camino.