Any question answered...

At first glance I thought tender writing was going to be about poetry :)

My experience is tech specs are written by engineers. Commercial teams interpret those into tender docs (poorly)

It's not uncommon for tenders to be weighted as much as 70% finance 30 technical quality.

Be willing to ask clarification questions (responses will have to be published to all parties)

Post tender award you can negotiate the hard detail of cost:quality and technical aspects.

Edit:if in doubt stick to the generic "we will comply" we have a policy at work of that being an automatic pass.

Tender writing - what's the knack? I never really do respond when I see them but was asked directly by a local authority and it's local which would be very welcome (usually I have to travel overnight to jobs). But I just can't get to grips with their way of weighting/percentage answers.

I can do the job to a high standard.
I can do the job at kind of whatever cost they want within reason, well under their proposed budget.
The tender questions are all written by someone who presumably has no experience of the job (wrong phrasing/terminology/odd expectations of what they think the job entails etc - very different from the brief document which is clear and useful).

Any tendererererers here? Any tips?

Almost always the tender is released after the organisation has made a choice on who they want to use, and the tender document will have been written in such a way that it favours one tenderer to the extent that (barring a real surprise) they're nailed on to win.

If you've been asked to bid, that might be great - or it might be that the authority want to find a couple of other responses in order to validate the one they've already got from their preferred bidder, as everyone else knows not to waste their time on a foregone conclusion.

Rule of thumb here - if you are engaged with the client before the tender document is released then it's worth some time, if you're only involved afterwards, throw a price at it that you'd be prepared to honour and move on with your life. If it comes in - great, if it doesn't then you didn't piss hours of your time up the wall.

Tender writing - what's the knack?

This sounds like a job for a Consultant!

It may need the same approach as applying for a grant, when you're a charity. There are people who specialise in helping charities by writing grant applications. Perhaps you could find one, and bung them a few quid?