Hey Neil.
I replaced my Odessey with a Westco and think it is better.
I put an Odyssey in my 1200 after BMW's originals packed in after a year and had no issues even when left lying for a few months. If you buy one of them it'll be a tight fit in the original mounting place.
I bought it from Motorworks or Motobins in the uk.
Bf.
Which model number did you get? I have a Westco on my K11 and its been great for 6 years, still reads 12.8V, so thinking of getting one for the GSA.
The model number is 51913, I bought it from tayna.co.uk and had no issues with them or the battery.