Ford Galaxy limited transmission available!

Hi
I’ve got a Ford Galaxy 2010 Titanium Tdcii automatic and have an error message on the screen saying ‘ Limited transmission available ,and the car is in limp mode .
After this happened the first time Ihad a mechanic do a diagnostic and he recommended a gearbox oil and filter change which hade done . The car worked perfect for about a week and then back to the same problem and limp mode and same error message . Also had the DPF cleaned to no avail at the same time
Another diagnostic and the mechanic reckons that solenoid Band C in the gear box are malfunctioning.
What is my next step ? As the car is 13 years old and only done 95000 so probably not worth a great deal of money although in amazing condition ,am I looking at a new automatic gear box at major expense or is there another fix .
Thanks for any comments ?