If it's only happening when coasting, the problem might be the ratchets/pawls in the freehub/freewheel. However, you can't assume that the change of bearings means it's not that. If the bearings were very tired, the races (the surfaces the bearings run on) may have been damaged to the extent that new bearings will not fix that problem.
Now, if you like bike maintenance, replacing the hubs (which will mean taking the wheel apart entirely and learning how to rebuild it) should ensure that the bearings there are okay. Likewise, dismantling the freehub/freewheel and replacing the pawls/ratchets (I can never remember which is which) should fix that if that's the problem. But, in reality, those two jobs will take quite a lot of time (even for a reasonably competent mechanic) and that's assuming you have/can get (and afford) all the tools, and can get replacement (compatible) components.
I prefer to make do and mend where possible, but reluctantly I would suggest that it might be cheaper, certainly quicker and definitely easier to replace the whole wheel and freehub/freewheel. Chances are someone here will have a workable replacement for £notmuch if you tell us what you have size-wise etc.
Plan B might be to ride this one in to the ground and if - once it's there in the ground and dead and buried - you like this cycling thing (fingers crossed), invest a bit more in something a bit better.
I made my first bike from a frame I found in the basement of a rented house. It was horrible, but I learned a huge amount from working on it. A frankenbike is a great way to learn.
And purple is the best colour.