Indexing is the likely cause; could be loose cogs but I think with grip shifters it's likely to be a cheaper bicycle that has a 7 speed freewheel and unless the freewheel is broken it shouldn't have this problem. Maybe the cogs are bent and/or the freewheel isn't on the hub straight?
Now a bit of a dig... you call them Halfrauds but still gave them further business. Now, I understand people have this united disrespect of certain large companies [Microsoft, Tesco,
Halfords, etc] but if you really feel that strongly then isn't it best to give the business to a company that would offer you the expected level of service? I buy stuff from Halfords as they are open 7 days a week, stock a fair amount and I could drop in to a local store.I would not get them to build/service a bike as I can do that myself and there are more than enough reports that their service is hugely variable depending on branch.