Equivalent of a John Lewis Natural collection Herdwick wool 8000?

After much procrastination and research I recently settled on a John Lewis Natural Collection Herdwick wool 8000 mattress, however before ordering I came across some negative reviews on 'Mumsnet' (not of that mattress in particular but of the JL natural collection - things like the mattress squashing down etc after a few months because of the use of two different types of springs inside - little as well as larger ones?). What is your nearest equivalent please? It would need to be from your Artisan range (having read your FAQ's) because we need a 6 x 7 foot mattress (180 wide by 213 long) to fit our existing slatted base sleigh bed. My husband is 6,3 and 14 stone and I am 5,8 and 10 stone so I think we are in the 'medium' firmness range. Many thanks