A quick bit of advice, please.

It's possible that in the next 12 months I will need a reasonably thought-through specification for an install in a new build venue in the East Midlands area.

This hypothetical venue would probably be a warehouse-type construction, with a main (single-level, high-ceilling) auditorium capable of seating up to 1000, and at least one further venue with seating for up to 250, with multiple smaller conference rooms.
Uses for both would be mainly live music and speech, with multitrack recording and AV distribution throughout the building also a requirement (currently running something similar in the current venue on Cat5).

I've been to several venues of this type (mainly churches) in which a decent rig (on paper) has been killed by the nasty acoustics of a metal box, and would prefer to try and get some acoustic design basics into any plans at an early stage, if possible.

This is way beyond my level of expertise, and I want it to be right first time. Does anyone have suggestions for individuals/companies with whom they have had personal dealings in the past, who would be in their element with this kind of consultancy job?

Feel free to email me if you don't want to be seen to be advertising on here. This isn't a tender, just looking for recommendations to help me narrow the search at the start of it all tim (dot) turtles (at) ntlworld (dot) com

Well, part of this will depend on your budget of course. But if the place has not yet been built, get a consultant in there NOW, and make it clear to the architect that they need to work with them.

You can replace the sound system, redesign it as needed, etc. But in order to redo acoustics you will need to modify construction, either materials, shape, whatever the case may be. If this space will be designed for multimedia, or audio, on a regular basis, acoustics should be very high on the list of priorities for it.

Seablade

Could do a shameless self-plug, but also knows he would hire in someone else for the acoustical side of things. I only know enough acoustics to design a system around it, not to modify it like a true acoustician would.

That's exactly what I'm looking for, Seablade. Although I doubt the materials budget will stretch to turning the building into a high-quality auditorium, I'm guessing that getting an acoustics engineer (I use this term to imply one who designs an environment with audio in mind, rather than one who creates audio while mindful of their environment) involved early on would probably save time and money in the medium term due to the reduced requirement for active or passive corrections at a later stage.

As you imply, there are many companies and individuals (your good self included and noted ) who could probably specify reinforcement kit for a venue such as this. An acoustic engineer is harder to find, though.
Any suggestions re: where to start looking?

My strongest suggestion? Spend several mornings/afternoons. Walk to some places with a similar goal, or that you find their acoustics particularly pleasing. Make a note of all the places you really like the sound of the place, and then contact the facilities manager or whoever handle physical maintenance of the space and see if they can get you the name of the consultant that worked on the space, or the architect and ask the architect.

On a consultation that is currently being installed, there was concerns about the acoustics. I picked a few places with similar goals(House of Worship in this case, with a full organ, primarily acoustic music, etc.) and went to listen to them. In doing so I was able to count out several possible 'acousticians' (And that term is used loosely in this case) because the sound was awful. But it also gave me some leads to follow up on. In the end, it was decided to do what I can with the system design and I didn't need to follow up on the leads, but I have them in case I need them for the future as well.

For the record however, the shape of the space can have as much of an impact as materials used in its construction. So don't count out changing the overall shape as well, not just materials. A box will always sound like a box, it just might sound like a metal, cardboard, plaster, marble, etc. box.

Seablade