My poor little Fabia has some really disgusting dirty stains on the seats from peevish owners (about 14 years worth).

Have tried the spray valet, worked a little bit. Tried a steam cleaner, has shifted some more filth, but still stained.

My local car wash charge a fair bit for upholstery washing, and needs to be booked, and wondered if there is any wonder product/technique anyone can recommend

Difficult to say without some pictures mate, but I've just done SWMBO's Seat Altea seats which had coffee stains on 1 and she spilt a bottle of mouth wash on the other as well as the usual grime because she hoovers it out about once a year and they came up like new. I sprayed Meguiars APC on with a normal hand pump sprayer, diluted as directed and let it dwell for a few minutes before wiping off with MF cloths from Costco, which are very handy as general purpose MF cloths. The Megs is expensive, but it's good stuff and you dilute it before using so it will last you a long time though others might be able to come up with an alternative

 

https://www.amazon.co.uk/Meguiars-All-Purpose-Cleaner-Litres/dp/B0006SH4GE?ie=UTF8&*Version*=1&*entries*=0

 

http://www.costco.co.uk/view/search?q=microfibre+%3Arelevance%3AlastCategory%3ACleaning+Cupboard

I've got a VAX which seems to get the worst out of most seats.  The state of the water in the bottom bucket bit is shocking when poured out in the sink.

+1 for meguiars apc 

one of the most useful cleaners i have ever used 

 

nip down to morrisons or where ever and hire a rug doctor machine for a weekend and give the interior a good going over 

 

quite cheap to hire 

Another vote for hiring a carpet cleaner. I cleaned the interior of my Fiesta that had gone mouldy and had some stains I didn't think would come out. They look really good now. Just make sure you get a small implement, rather than a big one for floors!

Cheers I did wonder about those rug Doctor thingys. I've used one on my carpet in rented flat it was revolting what it got out of what I thought was a clean carpet !