Hi scodav,
I'm currently building a fake chimney myself, you can see my progress here -
http://www.avforums.com/forums/home-...ey-breast.html
I've built the frame using CLS timber from B&Q (
CLS Timber (L)2400 x (W)63 x (T)38mm, 5014957047211), it's not the best for quality and if you spend a few mins when buying it to weed out the obviously warped lengths you should be fine. I've used the 6x100mm TurboGold screws from Screwfix (
TurboGoldĀ® Countersunk Screws 6 x 100mm Pack of 100 - Screwfix.com, Where the Trade Buys). They have easily pulled the frame together where they were a few pieces still out of line and they reckon you can get away without having to drill pilot holes (although I've still drilled holes in most cases, just in case of splitting the timber)
I've planned on making the faces from mdf but now I've got to that part the front face is looking fairly daunting so that may change in the next few hours!!
Hope this helps, I'm new to this myself so I'm sure some of the more experienced members may have a few different ideas!
Cheers,
James
p.s. Sorry about the long links, I couldn't figure out how to give them a short title!