How many recharge cycles has the battery done? Some people use their ebike's almost daily for commuting and could rack up 100s of recharges per year. Could the battery be end of life? It could be the battery is now struggling with some cells not working fully anymore.
Saying that the Suntour HESC ebike system has some common issues related to the torque sensor. Also bear in mind I think the hub motor is non-serviceable as it needs a special tool. So I would limit how much you spend on it. For example if they wanted £300 for a battery pack and the ebike is heavily used anyway I personally think I'd rather put that £300 towards a new ebike or ebike kit. You can get full ebike kits for £400 or less and if you stripped off the ebike components from the bike including the rear wheel they might fetch £150 or more on
ebay. You then replace it with the kit you bought which would have less proprietary components and likely a bigger capacity battery pack you would then have much more easily replaced and diagnoseable components.