I have fitted an "absorbed glass mat technology" after advice given to me by L.A.B. It is a Westco YB14L-A2 (12v14L-B). This fits the MK3 tray nicely.
Not only is there no vent for the acid vapour to spill out of as it is sealed and has no liquid, the battery performance according to specification blurb is superior to lead acid and Gel batteries, and although I cannot directly compare it, it certainly spins up my motor beautifully. 8)
As far as I recall, it allegedly has a much slower self discharge rate than lead acid batteries, and gives a higher cold cranking amp.
The only downside is it is a bit more expensive. :?