Hi,
Thanks for getting in touch and we're sorry to hear you're suffering from M.E, fibromyalgia and hypermobile back hips. It sounds like you're in a great deal of discomfort in bed which sounds awful. It also sounds like your cheap memory foam mattress may be contributing towards this. Let's see if we can help you.
We would advise having a look at our Artisan Naturals based on your budget and requirements. This model has a true medium feeling upholstery providing a slight sink but not too much. It also has the benefit of being constructed with 85% natural fillings. Those fillings not only last longer than synthetic fibres but also provides a cool sleep surface. This will give a softer sink for your hips to help relieve some of the pain.
Based on your weight you would only require a 1.2 gauge or soft spring tension. Anything more than this will be overkill for you. The spring tension is based solely on your body weight and not the overall feel of the mattress. The overall feel of the mattress is dictated by the upholstery or comfort layers that are housed on top of the spring support unit.

If you're looking for a softer sink then the Artisan Luxury model maybe suitable. Which has the softest top upholstery layers we create in a mattress, and I would recommend a soft tension spring in this model again based on your weight. The Artisan Luxury is 100% Natural fibre and like the Artisan Naturals is fully breathable allows for a cooler sleep surface. It really depends on your budget. Both would be far more suitable than the awful memory foam model you're currently enduring.

To summarise:
The Artisan Naturals - will give a medium feel (better if you're spending lots of time in bed)
The Artisan Luxury - will give a deeper softer feel (better if you're wanting a deeper more comforting nights sleep)
We hope that helps and if you have any further questions please get back in touch on 0161 437 4419
All the best
John & Ryan