Study Population Entry to the study will be offered to all women with OAB symptoms with impaired voiding on cystometry (flow rate less than 15mls/sec with volume of 200mls voided and a normal or high detrusor pressure at maximum flow), who have failed to improve with 2 different anticholinergic medications. Subjects will be recruited from the Urogynaecology clinic at Medway Maritime Hospital. A power calculation was carried out which showed a minimum sample size of 30 (i.e. 15 in each arm) to be necessary to ensure an 80% power with a significance level of 0.05.