plonked it all with Old Mutual (ISAs, S&S) which frankly is something I could have done myself
It's very easy after the event to say you could have done it yourself but did you know all the various providers available, their charges, their performance, etc? Would you really have even discovered Old Mutual yourself and if so, would you have chosen them over the several alternatives?
And what about the split of investments between types and then the actual choice of the fund in each category. Again, easy with hindsight and having seen what a professional has actually done, but I honestly don't think you'd have come to the same solution without a lot of research, time, worry etc.
He won't have just "plonked" it there - periodically, he will be reviewing the performance of the funds invested in, compared with performance of alternatives, the markets generally, etc., and will be advising when it's time to change funds/products, also bearing in mind your age to retirement (usual practice to move out of riskier funds as you get closer to retirement) etc.
I'm an accountant with 35 years of experience so know lots about finance, tax, investments, etc., but I still pay an IFA to look after my investment portfolio because I know that he has the knowledge of all available options, firms, funds, products, etc., so it saves me the stress and time of having to try to keep up with the research etc myself. For what he charges, it's well worth it for me. In years gone by, I used to do it myself, thinking I knew enough, but that lost me a lot of money in poor investment performance etc.