Dead!

A few days ago I went to go out in my MX-5, got the lights on when I switched on the ignition but just a lot of clicking when I tried to start the engine.
Put it on charge and used the old Suzuki Alto. In the afternoon I took the charger off and it started ok.
Yesterday it was dead again.
Battery is 5.5 years old, they generally come with a 5 year guarantee and this is the second battery I've had on the MX-5, which is now 11 years old at the end of this month. The first battery being the original.
Trouble is I don't use it much nowadays so it sits outside, looking pretty but feeling neglected when I roll the bike out. Since last August I've done 3 time the miles on the bike than I've travelled in the car, which has only done 1,400 miles since the last MOT in February 2017.
I'm afraid it's time to sell, I'll get it MOT'd, clean it up and see what I can get - with a 007 registration plate.
I certainly will miss it but sitting on the drive with Road Tax, Insurance and depreciation all ticking away I have to ask myself 'Is it worth it' - probably is for the summer but even then I don't use it very much, but I love it when I do!
New battery will be purchased on Monday if the new charge doesn't help, I'm using a different charger, a much more efficient one which monitors battery condition as well.