Young Texel x should be very very tender. Not, in my view, especially flavoursome, but incredibly tender. Ours is, anyway. I haven't tried Suffolk x Texel so I don't know what difference that makes. But breed does make a difference - our Charollais x lambs are succulent, sweet and full of flavour - but not quite as tender as the Texel x.
In your case, I would wonder about stress, speed of cooling at abattoir, correct carcase, hung for at least 3-4 days, cooking process and relaxing time before eating.
I have found that my own home-reared meat simply doesn't take as long to cook (and/or need as high a temperature) as the cookery books say. And I am anal about the relaxing time (in a warm but not hot place) - for chops too, not just joints - and I really do think it makes a difference.
I do hope it's cooking / relaxing - so that you can have some better meals off the remainder!