Faulty freewheel

[Haku]

Over the past few weeks I noticed when pedalling moderate to hard that occasionally the sprocket would jump, I thought it was the derailer not aligned properly and the chain jumping gears, but today I discovered what the real problem was - cycling home in near freezing temperatures (it's now below 0 outside) the freewheel in the sprocket stopped working properly, every so often it would just spin when I tried to pedal, resulting in the chain coming off and getting completely stuck between the sprocket & frame. I had to 'hobble' the last mile back, throttle on the flat & walking up the hills

Is the freewheel in the sprocket fixable or should I replace it? if I can, because of the power cable going in through the middle of it all. I have wanted a lower low gear for a while now to help with the really steep hills when I'm carrying extra kilos in the panniers.

 

[Mussels]

The freewheel is part of the set of cogs and designed to be consumable. From what I have read it's not worth trying to service the freewheel part, just order a whole new one with the gears you want and bin the old one.
If you want a megarange to provide a lower gear then your rear mech needs to be compatible and have a long cage, personally I ditched the megarange and got a smaller chainring instead as ideally the greatest choice of gears should be at the bottom. I found with megarange I was often hunting for the missing gear between 1 and 2.