First road bike: BTwin Triban 540 or BTwin Ultra 900AF ?

I test rode both of these models today. They felt worlds apart. The Triban felt so much more heavy and rigid and the slight difference in geometry made a big difference for my riding style. The lighter 900 was very quick off of the mark and felt very agile. I would probably change the cassette a few months in as the standard set up is more mountain bike than racing bike. Can't go wrong with Mavic wheels either! Personally I'm definately going for the 900 on the Bike To Work scheme.

Go with the 540. You'll end up getting a lighter/faster/carbon bike next summer and the 540 will be a better winter bike thereafter! 

The usual omissions from a full groupset are the crank and brakes.

If you're going with rim brake calipers, they're a very cheap item to upgrade.

For cranks...I've used Shimano's non-series cranks (the older FC-R565 and more current FC-RS500) and they're basically 105-level cranks with some more weight, due to the hollow manufacturing methods not being used. As long as you're happy with the gearing (these cranks usually come in 50/34T and 46/36T flavors), they're dependable workhorses that shouldn't let you down.

Needless to say, I'd pick the Triban 540. Good luck

The more relaxed geometry for the 540 is probably a better idea from an injuries point of view and its a much more versatile machine.   

If you're going to race then I'd go for the AF, for normal club riding the 540 due to normal seat stay rear brake and mudguard mounts

I'm a runner and I was out for 2 months with achillies problems due to cycling.  I would be extemely careful if you are injured and looking to cycling as a way to exercise through that.

My particular problem was right where the achillies connects to the heel.  My saddle was about 1cm too high which meant my foot was plantarflexed for most of the pedal stroke.  Your achillies is strongest under tension (tendon / tension - get it?).   When compressed you get wobble and, like running, are exposed to classic overuse injury.   The thing is, it's the kind of injury that build and builds over weeks and you don't notice it until it pops doing something mundane like getting out of the bath.   I strongly advise you see a physio and see what they have to say and if cycling is OK, get fitted by someone with some experience.

As for the bike,  I was about to buy the 900 AF.  I think it is great value and will take 28mm tyres.  The 540 is a more relaxed ride and can take wider tyres and mudguards.   Btwin also have an £800 carbon triban (560?) that seems good.  Decathlon should let you test ride the bikes assuming you live close to a store?

I had decided on the 900 as the frame is a bit better and I am used to the geometry.  In the end, I picked up a reduced canondale CAAD12 for £999.  I don't cycle race but might in the future.  Being a runner, I do like to put the hammer down a bit now and again and find (for better or worse) I can push harder on the bike.

Apart from the brand name snobs, I got the impresssion that decathlon bikes are well regarded for value.   I think get the 900.   If you've been running in the past, I'm not sure I belive that you wont be competitive on the bike, so that might a better fit

August is bicyce sale season so keep your eyes peeled for other brands.  You are shopping at the best time of year

Buy the 540 and spend the difference on pedals/shoes/kit. The wheelset is the same so you're paying an extra £150 for a stiffer frame with internal routing, and some extra bits of 105.

Not sure the 900 is that great value (they've got a carbon bike cheaper than that in the range). The frame is the same as the other AFs (starting with the green and black 700AF) and is well regarded. Not a fan of the chainstay rear brake though, not for British roads.

Are you likely to do your own maintenance? 540 is easier to work on than the 900 as cables are external. 540 is more versatile as it will take mudguards and from memory some decently wide tyres.

On bikes of this level/price range the thing that makes the biggest difference to performance is the wheelset and as its the same on both bikes I'd go with the cheaper one, particularly as you say you won't be racing.