Our cocker spamiel was diagnosed with stones and after a course of Hills prescription S/D (tins)
which has now cured him he was put on Hills Urinary Maintenace (tins) which we have been told he must be on permanently. Although we have pet insurance which covered the cost of treatment it does not cover dietary requirements.
This is proving rather expensive. Does anyone know of any alternative, preferably dry ?

If you have a look at this site it would appear the best form of prevention is not by diet at all but by regular monitoring for a return of the infection.  You might want to discuss this with your vet and enquire how the monitoring might be conducted, for instance you could purchase urine strips to test your dogs urine at home on a regular basis.