Is the jump from £1000 to £2000 worth it?

£3k is the cut off to diminishing returns.

If your bike is decent and you like it then 1st step is to upgrade wheels, £200-300 or less 2nd hand will make a big difference, easily best bang for buck. Otherwise I'd say it's not worth upgrading as your bike is a new model and the jump up isn't enough to notice a significant difference. So wait either until you can afford the £2K + bracket, or until technology has moved on to a point that allows a smaller price jump to be a significantly better bike. Best to buy end of year for the bargains too.

It's definitely a case of diminishing returns for your money. However, what else would you spend your money on and would it give you as much pleasure as a new bike?

It won't make you noticeably faster in and of itself. If you want it though, then do it - loving your bike is a big part of what gets you out on the road, buy something that makes you want to ride it. I've got silly expensive bikes that I love riding, I've also had cheap Chinese open mould stuff that was technically brilliant but I just didn't care about so they sat there unloved

Do it!
I've bought two 2nd hand carbon frames off eBay. The first was my Wilier Izoard, which I've still got, as when I first rode it, it was a revelation as my first carbon bike. It's got Ultegra on it, but I'd happily use 105 R7000 on it. It's been to the Alps & Pyrenees on a number of occasions. The difference between Alu and carbon is huge in my opinion, and I'd always opt for carbon.

My 2nd is my current training bike, which a Kinesis RS carbon frame, cost £135 on eBay, duly fitted out with spares from my parts box, except the shifters which were also 2nd hand 105 R7000 for £75. It's a superb bike for what I paid.

Might just be me being paranoid, but I'd be wary of buying carbon 2nd hand (especially from ebay) unless I personally knew and trusted the seller

If you've not already had one, a proper fit might be a good investment. Then when you've got new shoes, insoles, stem, pedals and saddle you'll have less burning a hole in your pocket

I doubt a £2,000 bike will be that much nicer than your Allez. Have you considered some new wheels? And lighter, fast-rolling tyres?

The first thing I'd do is swap the Tektro calipers for 105s and fit some Kool Stop dual compound pads.

It's an interesting price point as it bridges 105 and Ultegra territory especially if you look at last years models in sales.  However there's an arguement to be made that Ultegra's not worth the uplift as 105 is so damn good.  Your Allez is going to be compromised by the Alu frame and it only has half a 105 gruppo on it, plus you're locked into rim brakes.

I'd be tempted to retire the Allez to winter bike status which it will be fab for and buy yourself a summer pocket rocket or something that floats your boat.

ebays a great option for secondhand - a grand will get you something with a great frame,wheels and full 105.  I got a Fairlight Strael for instance at that price.

To be honest the best way to go is second hand . You get way more for your money.

research is the key . There are some real bargains.  

what are you looking for? More speed, comfort, enjoyment, pride of ownership, reliability, convenience, durability.. ? A lot of people will deny it, but deep down some of the pleasure just comes from owning and riding a fancier machine, and why not?