Search marketing is typically an ongoing process. It is never 'finished', even if its just on-site SEO of content that will never change, there's every chance that further tweaks will be needed sometimes, and that things can still be improved. When you add in off-site SEO and PPC stuff, you immediately hit areas where there is considerable ongoing work even just in keeping an eye out for click-fraud, changing bid strategies of competitors, etc.
There are some companies who avoid retaining an SEO or agency month-on-month and prefer to do everything on fixed cost, fixed scope projects, but I have never, in 18 years, seen one doing well. (I'm not including companies that have ongoing in-house SEO staff and outsource only fixed-cost projects, of course).
If you are capable of managing the ongoing refinements, monitoring the PPC campaign daily, adjusting the bids, etc then you will count as that in-house SEO and could probably cease the monthly contract by puting in the hours yourself.
To be honest, it sounds like you have a very minimal commitment to SEO, as £250 per month is a very small amount. That's like 2-3 hours of work out of every 30 days you are paying for there.
SEOs fall into two very different camps. There are the cheap SEOs, and there are the good SEOs. If your SEO is good, the reason it will be going slowly is because you are not paying the going rate for even one day of work per month at £250. None of the firms I really trust even do a half-day at a rate that low.
Now, if the results are good, keep on with it. The old saying "if it ain't broke, don't fix it" applies to contracts too.
If you cancel the ongoing work, and you are wrong, your rankings and/or conversions fall, well, that SEO may never want you back as a client later. Other reliable companies might be wary of taking you on too. You'd seem a bit flakey and uncommitted. One of those bosses that always second-guesses the experts and tries to save money even at the expense of wrecking things. Now, I'm not saying that's what you would be, but its a risk you could be seen that way.
If the SEO has been begging you for the commitment to do more, (asking for more hours in the budget, etc), then seriously consider listening to that expert you hired, and do a short term trial at a higher commitment to speed things up.