The scrap metal man in quay lane Gosport always seemed to be ok, but like you say if you don't know what they are worth then how do you know they are not taking the piss!
Ironically I have just scrapped a Mitsubishi CAT today and got £40, scrap has dropped to a 6 year low with copper now at £3000 per tonne , 2 years ago we got nearly £4500 per tonne scrap copper
Got £24 for one the only number I can see anywhere on the is inside the cat on the honeycomb on the outlet side of it. Can't remeber the number now but will look tomorrow.