Thanks for the good details BexTech.
My problem is that I'm currently already using;
1) 18866 (but a peeved since they've put up their connection charge a lot recently without telling people)
2) Talktalk (calls only not line rental)
3) BT for landline, (but don't get free caller display as all calls not on 18866 go via Talk talk so not eligible for it)
4) Plusnet for broadband (who I've been happy with over the years)
This amounts to 4 seperate bills (plus mobiles) and so even if I'm getting the lowest rates, it's difficult to tell how much useage there is on each type of call so can't tell if it's being used too much. ( A bit like buying something in the shops at 50% off because it's cheap, but if it wasn't actually needed, then it's actually cost the 50% rather than saved 50%).
My thoughts on solutions were initially the Talktalk package (but am glad I didn't as it appears people aren't happy with it.)
Another solution was to try 18185 etc, but that'll mean yet more accounts to sign-up to and not less, plus trying to remember when to use which one (or more importantly when the family have to remember

) but it's not discounted just yet.
My other solution appears to be PlusNets offering which I'm currently investigating and have already asked them about the ability to call indirect access code like 18866, the cost of caller id, and (believe it or not, there's no mention of the 1571 answerphone service). Answer so far - wait till the 4th July. So these things are going to be part of my decision making process.
I might try them anyway, as long as they have 1571, (look at this I'm talking myself into it as I type

) as it'll be easy to go back to BT if necessary (only 1 months notice), and their £15 option will give me free landline calls anytime within the UK (& daytime is an important one here to my wife), plus free calls to a number of countries, one of which I have family in. This should leave us with just o870/0845 and mobile calls which we try to keep low anyway, and the mobile rates are cheaper than 18866 for the majority and definitely cheaper than talk talk).
I'll let you know how I get on if I do actually sign-up
(Other advice is still very welcome though from anyone)