If the big prizes are fairly evenly spread around surely you would expect to see a couple who have the maximum holding and have held them for say 5 years to have at least won one prize bigger than £100

I disagree.

IIRC people with the max holding can expect to win about 12 prizes a year on average (although for any individual this can vary widely).

Only 1 prize in every 500 is over £100.

So on average those with the max holding can expect to win a bigger prize about once every 41 years.

Even then the year in which the bigger prize is won could be any one of those years. So it might be year 1, it might be year 20, it might be year 39.

But as the draw is random one can't expect that all max holders will win a big prize once in any 41 years. some max holders will do better than this and some worse. Some may win a big prize every 10 years and for others only every 100 years.