I had the same problem yesterday, i.e. cold morning resulting in thick oil and a 3 year old battery that was not really up to the job even when new.
The enging turned over a few times without firing and then the battery was drained to the point where it would not turn the engine over any more. This is despite being hooked to a C-TEK maintenance charger all the time it is in the garage. I connected a conventional charger to the battery and after 4 hours it span the engine over and it started easily.
I also think the question of the limited CCA capacity of the standard battery is the crucial factor and I wonder if anybody has considered fitting a second battery (of the same type as the original) and connecting it with heavy cables, in parellel with the existing battery, to effectively provide a total CCA of 600 Amps.
I think there is room under the bonnet on the passenger side to fit a second battery tray and the associated battery retaining strap and I have just been quoted £48.00 for a replacement battery so replacing the pair in the future would only cost around £100 and they combined would give the greater CCA needed.
I would welcome other member's thoughts on this.
The Westco battery I have fitted has CCA of 400A and whilst it did not start instantly this morning did start quite easily. This battery is essentially the same dimensions as the Banner battery. However there is plenty of space above the battery so a wider battery could be accommodated by positioning it higher than the existing, this gives many more options for choosing a battery with greater capacity. I did not go this route because I thought the Westco would be direct swop but as it turned out was a few millimetres wider and required an adaptor making anyway.
