Is there a penalty for not meeting agreed exchange date?

[rumbelina]

I don't mean pulling out completely but if we agree exchange date with our buyer's solicitors can they then change it?

Ifaik, the date forms part of the contract and is legally binding. However, delays happen for genuine reasons, and if you try to hold your buyers to an exchange date they really can't make, you risk losing the sale altogether.

In England no penalty as such although if it is a new build I think you may find a clause saying they will remarket the property, or you could lose "extras" like carpets. exchange dates move until exchange actually happens although completion date is usually fixed (again may not apply to newbuild where completion date may be distant) Scottish system differs.