Thank you to everyone for all of your help
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I do clean out the tray regularly and scoop every day, however, Charlie's outdoor antics are not so much these days and he doesn't want to go out much at all. he's become quite lazy since his neutering. Because of that he's often in all day and even though we have several trays dotted around, the first person home is greeted to a noxious aroma!!!
The bicarb is a good idea, I'll give that a go as that obviously isnt harmful to the little blighters. Hadn't thought of that one, so thanks.
I too scoop regularly, morning and evening and every time there is a 'solid'... I use wood based litter as I like the au naturelle element, but might start mixing it with a clumper and one that has a deodouriser built in...another idea i didn't think of!!!
We have a couple of trays, but I also like the idea of having a few more to swap litter into when cleaning the others. I tend to clean with boiling water, washing up liquid and then a blast with pet friendly disinfectant too. I think the smell builds up because he is in most of the day, the doors and windows are shut etc.
I agree that him going indoors is good due to problem detecting etc. but my other half was a bit anti cats anyway until he met Charlie. Charlie is 'His' cat (I believe there is at least one for everyone!). I dont want him to feel resentment towards him at this stage!!!
We dont have a garden, we have a flagged yard, so it could just be that he doesnt have enough of the right material to make him go outside (although there are plenty of options around us, but may have been claimed by other cats). This is going to change as i'm digging up the yard and putting in flower beds etc. so he might feel like going outside when that happens.
Many thanks to you all for your suggestions, really appreciate your ideas
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