Hi Purple,
It does not seem likely that 100% of the billable time accrued with the solicitor was spent giving wrong advice; but in principle, yes, there should be a significant discount if the solicitor is charging for giving advice on a relatively straightforward employment law issue and if the solicitor had that advice totally wrong.
The 'painter and his mate' analogy is not really apposite. A solicitor will hire a barrister to advise when the matter requires more research and\or more detailed knowledge than the solicitor has - the barrister is more like a specialist; doctor calling in a consultant (or painter calling in a guy who specialises in murals, if we must have that analogy) would be a more appropriate analogy;
Nevertheless, the solicitor should not have given an advice letter which was totally incorrect. I am basing this to some extent on your assertion that the errors were relatively simple - i.e. based on 'not much more than you can read on the employment forum here' , but I think you need to remember that your own knowledge levels might be relatively high, and your view on what is a simple error might not be totally objective as a result.