Thank you for your question.
Your employer shouldn't be with holding your P45, but I imagine when starting with your new employer you filled in a form P46 is the absence of having your P45.
Your new employer will send the completed P46 to the tax office that deals with their PAYE tax, and the tax office should review your affairs.
If the tax office haven't received a copy of your P45 either, they will write to your former employer for details of your pay and tax to date of leaving.
But as you advise that you have been to a tribunal for outstanding salary and holiday pay, I imagine the non issue of the P45 just summarises the battle you have had with this former employer.
I will advise that emergency tax, still gives you your tax free allowances each week/ month, but just doesn't take into account any previous pay and tax deducted. If your employment has been constant - in other words you have been employed every week/month during the tax year 6 April 2008 to 05 April 2009, then the tax you have paid will be almost accurate.
Any monies that the tribunal recognise as being owed and that your former employer is instructed to pay you, are taxed when they paid, rather then from when they are due, so to sort out your tax up until 5 April 2009,if you have payslips that accurately reflect what you received (rather than what you were due) then you can photocopy these and send them to your new tax office, advising that you haven't been able to get your P45, and the tribunal pending. If you send this with your P60, that your new employer should provide you between now and mid May, the tax office may be able to review any repayment you are due.
Your next salary from the new employer will see emergency tax being lifted, as its a brand new start to the tax year and allowances on a cumulative basis resume.
Hope this helps