Okay, the low battery warning came up, so need a new fob battery. No problem, as second key unused, and working. Tried, by method in handbook, to open fob and check battery size, and can’t open it. Very slippery plastic, and can’t get it to move. Any tips for an old codger with arthritic fingers?

The method described in the handbook is hopeless! Try reading this thread

 the pictures show exactly what you are trying to do to open the fob

Video 'how to'.

 

https://youtu.be/jBpux8Oyguw?feature=shared

The OP doesn't say whay type of fob he has: if it's the type with a flip-out key then the procedure is completley different. Flip the key out & they just use a wide flat bladed screwdiver in the slot the key lives in & carefully prise the cover off. The manual says use your thumb but that didn't work for me.

P-J has got the right fob, and the video he referenced has shown me how to do it - hopefully I will be more successful than so far.