Sturdy office chair (Kensworth LU6)

My 35 year old office chair has finally given up on me! The gas lift mechanism no longer holds it up so I'm desperate for a sturdy office chair please. A plain dark colour would be lovely, as would material rather than leather/plastic, but I'd be grateful for an offer. I'm quite big (15 stone these days) and due to disability have a habit of dropping into a chair rather than lowering myself like a dainty elephant. Collection during the day would be easiest for me as I go to bed early after taking medication, but again happy to br flexible around you. Thank you!