without knowing how much tax & national insurance you pay its hard to say but your gross pay per month based on a 40hr week would be �968.75 ... if you take off 25% for tax & n.i (a bit too much, but as a ball park figure) you would end up with �726.56 per month / �181.64 per week based on a 4 week month ...
Pro rata really depends on the full tme hours. The above post was based on 40, but some are 35, some 37 and a half. Basically you need to find out what the working week is to get an accurate idea.