"On a related note - would it be standard these days to get a full estimate for all work that the solicitor would do involving a separation. How nailed down are the costs that they can project?"
A solicitor might be able to give a reasonably accurate figure based on fixed assumptions. For example, if both parties have been through mediation, or have otherwise agreed all their differences, and simply want their solicitors to formally implement the agreement, it should be possible to give a reasonably accurate estimate (big generalisation follows. When a client comes to the solicitor with any litigation matter and says that it is "straightforward", it almost never turns out to be. There is a strange divergence between client perceptions and reality; the client who tells you his\her case is straightforward tends to be unduly optimistic; the client with a genuinely straightforward case is unlikely to tell you how straightforward it is)
However, if there are arguments of any substance (particularly issues of custody and access) it is almost impossible to give an accurate estimate, and a straight hourly rate of charge would be more transparent (though the figure could rise to a very high level by the time litigation is finished)