DD complaining her clarinet not up to much but as she mainly plays sax these days & we just bought her a new one of those she will have to wait!
I dont think you would get a new clarinet that was worth trading up to for Â£400 though - guessing Â£600 plus as absolute minimum.
Should be able to get spec of he second-hand one to check current cost at least? Also much better to buy from reliable source than newspaper ad etc. Do check when it was last serviced as that can be very expensive.
Do you have access to rental scheme in the short term if the capital outlay isn't viable atm? Some music shops will do hire with option to purchase later so what you have paid in hire charges isn't all lost.
Unfortunately music seems to be an expensive business! Ours have had instruments for Birthday/ Christmas presents in past but as they move up the levels the cost of instruments rather exceeds normal present expenditure