workmen etiquette - changes to original job / quote

[PinkOboe]

We booked our workmen in a few weeks ago. In the interim we have done a few parts of the job ourselves as we unexpectedly had a bit of time.

they are due to start next week. do i phone them up and tell them what we've done, so they don't come expecting x amount of work, for which they've allowed a certain amount of time for and will expect paying for?

i don't really get how it works? i don't want to diddle them if they've set aside time and are expecting to be paid, but our budget is tiny and in doing a few bits ourselves we should be able to reduce what we've asked them to do

Definitely give them a ring ASAP to let them know.
Hopefully they will be fine with it but, as you say, they may have now set aside more time to do the job than required.
I think the earlier you tell them the more likely they will be able to offer you a reduction on the quote.

Yes, keep them informed.
Might they have ordered parts or materials or be bringing them with them? Even if not, the sooner you tell them the better.