Your bike is now 18 months old and it is possible that there has been some battery deterioration. The good news is that the batteries on the Model 30 are relatively cheap- another test ride should about cover it!!!
However I understand that London has been very cold in the last few weeks and I suspect this will have had a big impact on performance. This would happen without any battery deterioration. If you can wait out the freeze you might find you don’t need to change the batteries at all.