Redundancy pay in lieu of notice and tax/new job

jj1234

My partner has taken voluntary redundancy from a job he was at for 15 years. He has seeked advice from a solicitor and an agreement drawn up etc and is recieving his pay out in a lump sum. He is entitled to 12 weeks notice which he hasn't had to work only the first couple of weeks and his employer is paying him through the usual wages monthly until this notice period is up. He was fortunate enough to get a new job straight away which he starts on monday, however, because he has 2 more monthly payments to come from his old job his old employer can't give him his p45 yet. How will this affect his wages in his new job? Will he be taxed heavily as a second job or does he need to give his new employer the p45 straight away?

Thanks in advance for your advice