At current exchange rates (which change every day) £25 is about 31€.
My favorite little hotel, the St. Andre des Arts, which is in in almost the exact geographic center of Paris in an area that is very lively and filled with cafes, bars, restaurants and shops, has four person rooms for 124€ which includes breakfast and all taxes. http://www.france-hotel-guide.com/h75006saintandredesarts.htm
The Hotel des Allies, also very well located not far from the Luxembourg Gardens and the Sorbonne has doubles for as little as 55€ and some triples for 88€. However, no breakfast is included and shower facilites are shared in some rooms. http://www.alliesparishotel.com/
The Hôtel Tiquetonne, on the right Bank at 6 rue Tiquetonne, is a well known cheap sleep, with doubles for 50€. They have no internet presence so you have to call or fax for reservations. Phone: 01-42-36-94-58 Fax: 01-42-36-02-94
The Auberge Internationale des Jeunes is a hostel, not quite so well located but not that far from the center either. Dorm rooms with breakfast included are as little as 14€ a night per person. http://www.aijparis.com/framea.htm