ive just been looking and this is what I have found so far...

Campanile Northampton - £49 per night, £147 for 3 nights
Travelodge Upton Way - Not giving any availability anymore
Premier Inn Harpole - Fri/Sat/Sun = £48/£60/£29 So £108 if we leave Sunday, or £137 if we leave Monday

and what is looking like the winner for me atm...

Ibis Hotel Northampton Centre - €119 for 3 nights (€129 incl breakfast)
Parking is £5.50 per day though, but will still work out cheaper than the rest...