Go through the insurance! if you were on the inside lane, and you have damage to the car that proves you needed to evade him (crashing into the roundabout) Then they have very little to go on as evidence of your misbehavior.
Do not go privately in this instance, as you will be out of pocket.
put it this way: you have more evidence to suggest you were crashed into, therefore their insurance has to pay out, not yours. They would be eligible for all costs.
If they do argue the toss and somehow you end up being blamed - push it back to your insurer, and get them to dispute this, giving them everything you can all descriptions and any witness details, if your insurer can argue right, the cost will go back in your favour.
Worst case scenario, they decide your 50/50, each insurer is then eligible to cover the costs, in which case you discard the claim and advise you will repair yourself. No damage done to your insurance, other than the fact you were involved in an accident.
Failure to notify your insurer at least is worse, if they start to play the whiplash card, you could be done for failure to report an accident appropriately, your insurance may be cancelled, and you may end up in a lot worse trouble...
It sounds to me like you have an excel;lent chance of getting what you deserve (your car repaired, and with no cost to you!)
Whilst everything is fresh in your memory, you need to write a report detailing everything that happened. I would advise, that to keep it in your favour, you report how you were driving, and he was for some distance before the roundabout. for example, if you both followed the same direction for 3 miles before, was he driving aggresively? was he tailgating? was he driving fine and this was just bad luck? you need to detail anything you can remember - in fronr of you there was a car X (fiat punto for example) he was driving behind an escort van, detail the weather, temperature, any roadsigns around. Everything you can - revisit the site of the accident and take a wander to refresh your memory.
If you can provide a thoroughly detailed description of the event, the insurers cant ignore it, if its your detailed story vs his "well, he drove into me on the roundabout" whilst you have details of damage to your car to suggest you where on the roundabout, then you have a much stronger case. Also witness, how many other cars can go around that roundabout, do you often see two cars, side by side without issue? is the roundabout sufficient in width to accomodate this?
Detail is the key to this and you need as much as possible to ensure you are not out of pocket at all...!
I had to do all of this after my accident, I was hoping that the responsibility would be passed to the other driver, as he slammed his anchors on in front of me, however in the end, all the evidence pointed strongly in his direction, however, because I was behind him, the responsibility ultimately fell to me. I took the initial claim that was 100% my fault, and my insurers agreed by the end it was more 51% my fault, and 49% his, but not enough to go for a split claim. so prepare to argue your case, ,and also - prepare for it to go on for some time, but do not give in, do not allow them to pass the buck to you...