Hi Graham
I am in more or less the same boat and indeed was due to pay the 15% in July 2008. As the guarantee is still not forthcoming and PW have not been chasing me for the 15% installment that is where the matter rests & in fact I am not unhappy at this stage to leave it that way. Your solicitor is quite right - under no circumstances pay another penny until you get the guarantee. Call me an old cynic but I am not overly convinced that the guarantee offers any huge protection in the current financial circumstances anyway
I would suggest that you email PW (make sure you save a copy of the email) and advise them that your solicitor has told you not to make the 2nd payment until such time as the guarantee is received. This will put the ball in their court and there should be no possibility of them claiming that you simply didn't pay and seek to apply interest from the due date.
Hope this helps.
Dave