Derek, it's a bit of a shock isn't it?
You'll see how I felt if you've read the link that Joe gave you above.
A pair of outer sills will cost you roughly £150 from fleabay. That's where I got mine and they were perfectly adequate.
The problems may start when the repairs start and more ugliness is uncovered.
As Joe says, they are structural and are an integral part of your car's bodyshell strength, being of monocoque construction, the integrity of the whole system will be compromised.
I know you are in Cheshire but if you'd be prepared to travel to Stafford I could put you in touch with the chap who did mine.
If you are interested, PM me and I'll let you know his contact details... also the price he charged which, for a week's work, was excellent value.