Yes, that's about right, however, now I have new job, this will take me over these thresholds so no need to sign on or anything I am assuming?
No that is not correct because all of the contributory benefits depend on National Insurance contributions which are calculated separately for each job. If you had 10 part time jobs paying £100 for half a days work you could be earning £50k a year and paying higher rate tax but still not qualify for the basic state pension.
So as current employer IS registered for PAYE, I can ask for my tax code to be split for new job?
Yes, but beware this is unlikely to be in place for 2012/13.
Should I be asking for the same gross amount as current job as they too want to pay me £10 nett (got contract yesterday, it says £xxxx gross per annum, anticipated to be £10nett per hour)
If that is what the contract says I think it is fair to assume that that is what they want to pay you. If you want them to pay you more you can try asking them, but if I were you I wouldn't try justifying it on the basis of what you are going to take home net because your tax affairs are no concern of theirs.