Oh bless him :) You've done well to get the mileage out of that - just watch his weight as the Maxi Cosi seats are outgrown at 13kg (most of them) or 12kg (Pebble plus) depending on which one you have.
Did you know it's much safer to keep them rear facing in the next stage of seat as well? Luckily there's lots of choice for rear facing Group 1 seats on the market nowadays and they don't cost a huge amount compared to a good forward facing seat either. Do you know which car you'll be getting - will it have isofix and/or top tether? Any storage compartments in the back footwell? Are you likely to be taking the car seat out and installing it in other cars at any point or just leaving it in yours? Three door or five door? Those things will all affect the decision you make. Best thing to do is to go to a retailer, ideally a specialist, second best a nursery store such as John Lewis or Mothercare, and try a few seats out in your car to see which fits the best. But these suggestions might be a useful starting point.
If you're on a tight budget I'd look at the Joie seats - Tilt, Steadi or Stages. (£70-120) Or if you are still on a budget but want something with a few more features (more recline/upright options, spinning for ease of getting in/out, option to be rear or forward facing, isofix fitting) you could look at the Joie 360 Spin. (~£200)
If you have a bit more money to spend and you like the idea of a rotating seat, you could go for the GB Vaya, Cybex Sirona S, Britax Dualfix or Nuna Rebl Plus. (£200-300)
The two highest rated seats for safety over the last few years in this category are the Britax Swingfix (which is essentially the Dualfix i-size in rear facing only mode) and the Nuna Norr. However if you really want to maximise safety, there is the possibility of continuing rear facing right up to 25kg (about 7 years) with a Swedish style seat which are available from specialist retailers. These give a bit more legroom plus will las longer than the Group 1 seats but price is comparable depending on model (£180-350ish)
If you'd prefer to look at something forward facing I would look at the Britax Advansafix or Triofix (£150-250). The Advansafix turns into a very good high backed booster so will last him until he doesn't need a car seat any more.
The only seats I'd specifically say to avoid are the kind with rucksack style tightening buckles on the straps. These are all made by a company called Team Tex, sold under various brand names with various covers (some novelty e.g. Disney/superheroes) but they all perform very poorly in crash testing above the legal minimum. Also a note about Cosatto at this stage - they do lovely fabrics but their seats perform poorly on crash tests as well. The infant seat is OK and so is the high backed booster, but the in between ones are not great.