Use a tax calculator (there are lots online) and for 2016/2017 tax year put in half your annual income, as you will only work half the year.
That will still calculate your income tax liability correctly - it doesn't make any difference whether you earned £X in 6 months on a £2X annual salary, or £X in 12 months on an £X annual salary: your total annual income for that year is still £X.
Putting in £21400 as your annual gross pay into http://www.uktaxcalculators.co.uk/ (and making various assumptions about your age, marital status etc.) gives total income tax liability for the year as £2080.
National Insurance is a different matter as that is calculated weekly:
- 12% of your weekly earnings between £155 and £827
- 2% of your earnings above £827
Your weekly gross pay would be £823 (£42800 / 52). Putting this in to the same calculator as the weekly pay gives a weekly NI liability of £80.16; across 6 months (26 weeks) this would therefore be a £2084 NI liability for the year.
So, for 2016-2017 your net income would be £21400 - (£2080 + £2084) = £17236.
Please do visit the calculator yourself and check my assumptions as these figures may not be exact if you have other income, allowances I don't know about, etc.