I assume you mean a FT person is on £1700 per month - but you work only 30 hours per week of the FT contract?
If so, your gross monthly earnings = 1700*30/37.5 = £1360 per month
National Insurance to pay each month = £100 approx (11% of your gross earnings that are above the Primary Threshold for NI)
Income Tax to pay each month = £165 approx (assumes you have a normal single person allowance of £6475 - and hence a tax code of 647L).
Net pay is about £1095 per month, unless you have to also pay into an employers pension scheme.