Gaz
I use JCT contracts all the time. I always work through an architect, who sorts out the JCT. On a contract of 100k my penalty clause would be £100 per week, though I have had £40 per day. There would be no bonus for completing early, why should there be. I should be able to progam the job properly and not be far out on the completion date.
If the builder says he can finish in 4 months for 100k then that's what he signs for, you don't reward him for finishing a bit earlier or we'd all be adding six weeks to the program. The other side of it is, if the builder finishes in two months you can't say 100k for two months work is too much, the price was agreed by both parties and the contract keeps you to it.
Personally I prefer using contracts, and penalty clauses are rarely invoked. If there's a delay beyond the builders control, such as bad weather or unforseen extras all these things have to be taken into account.
You have to remember a clever builder will be covering himself. If I thought a client was likely to stop £100 of me for being a week over, then he would be charged top whack for all the extras during the job, and the prelims would be added for the time on these extras. One week of prelims can cover a few weeks of overun.
Try it at £100 per week, this should be acceptable to most builders, to me £500 for five weeks over is nothing, especially on £100K. I'd be more worried about five weeks extra labour costs. If they don't want any penalties or contracts at all it could be that they've not enough confidence in their ability to work to their program. Or maybe not enough funds, with a JCT don't pay any money up front just pay every four weeks on work completed.
My completion date never ties with my program I always add time on, just in case, and I've never had any penalties.
At the end of the day penalty clauses are a minor part of what the contract is about. It's more about what you're getting for how much money. If it's agood job, done how you want it would you really worry about a few hundred pounds for it being a few weeks late.