What helicopter?
Being based in the UK myself, most of the year my starts are below +15c.My EC120 is not flown every day but at least every 4 in winter and most days in summer. No GPU used unless we have a problem starting, and I don't carry a GPU to land away. I haven't had a problem with either of 2 EC120s unless (1) battery not fully charged; or (2) cells dead in the battery; or (3) dirty / lose / corroded connections.
I have also seen problems with EC130 earth cable "wearing through" (eventually breaking in flight) which first manifested as reduced starting performance.
I have also experienced dirty connections causing voltage drop, as noted above.