Terrible news Simmy, glad to see you're looking at doing something productive.
A friend of mine has seen a few members have heart problems and has taken it upon himself to shed down on the weight. He got up to 27 stone. He was an active worker and bowled 20 overs of spin on a Saturday, 8 overs on a Sunday and played as a goalkeeper in the winter, thus he diet was the wrong side of appalling.
Over the last 9 months he has shed about four and a half stone. Although this is not a fair reflection as he is putting on muscle as well, in pure fat lose, god knows what he has lost. It is his intention to get down to 18 stone. He is currently challenging himself to lose 56 pound in 112 days (3.5 pounds a week) and halfway through he is well on course. His aim is to be 18 stone by the start of the next season.
His advice, set goals. Its what gets you through it. Make them realistic so when you fail on a weekly basis you feel guilty, when you beat them you feel like you're really getting somewhere.
PS. I agree with all of the above. 80% = diet, 20% = exercise in my book.