I think some male cats have a problem with spraying if they are neutered too late, ie: they have already reached adulthood. It is territorial and he probably feels threatened and trying to make his mark. It may help to speak to a holistic vet and try out some herbal remedies. You may need something stronger than washing powder to rid the odour, here is a link for the feliway that may be cheaper than the vets.
http://www.vetuk.co.uk/index.php?main_page=ind ex&cPath=149&zenid=ef2713be5b7d618d6a023031e81 a979e
I had some help from a lady called Jane who makes up herbal tinctures, her web site is down at the moment (sun essences.co.uk,), but her mail address is
[email protected] - you could ask her for some advice with this behaviour.
As regards the odour, try 'check' from stock nutrition, it is only �5 for a small bottle, which you dilute and it lasts ages, smells like strawberries too.
http://www.doghealth.co.uk/hygiene.htm#Check.