There's a variety of Bosch batteries, from the fitting guide above, it looks like there's these three that are suitable, not sure which one is the "Silvertop":
95Ah 850CCA model S6 002
100Ah 830CCA model S5 013
74Ah 680CCA model S4 008
I'd prefer the Ah's, rather than the CCA's
I'll check which one of those Bosch's GSF does for £75-ish... otherwise there's the Varta H3 for £100-ish...
The link before to eBay looks like the Varta version ofthe "Bosch S5 013" is a "VARTA H3 (019/017)"