Nationwide
Do I have to pay the excess towards a Third Party claim?
No, you only need to pay the excess towards your own repairs.
http://www.nationwide.co.uk/insurance/car_insurance/claims.htm#ihavetopay3
Direct Line
With Direct Line Car Insurance, you only pay the excess amount when you make a claim under our Comprehensive and Third Party Fire and Theft Car Insurance policies. Excess does not apply to Third Party Only.
http://www.directline.com/car-insurance/buyers-guides/excess.htm
From Defaqto
For third party, fire and theft insurance, be aware of the excess that is applicable for fire and theft claims.
Policy excesses are the costs you may have to pay towards the repair of your car.
https://www.defaqto.com/guides/insurance/car-insurance-guide/
The AA
What is a policy excess?
An excess is the first portion of a claim which you have to pay, regardless of blame, when your vehicle is repaired or written off, and is individual to each policy. You will only have to pay your own excess and not the third party's excess.
https://www.theaa.com/insurance/car-insurance-claims-faqs.html#Whatisapolicyexcess?
Honest John
If you have comprehensive insurance then you only pay the excess on your own claim, not the third party's. Your insurance has to completely indemnify the third party. These days, even being involved in a claim can hike your insurance, whether you have no claims discount protection or not.
http://www.honestjohn.co.uk/askhj/answer/19710/car-insurance-excess
Adrian Flux
What is an excess?
An excess is an amount payable by you in the event of a claim against your policy for your own damage/loss. Details of specific excesses relating to your policy can be found on your policy schedule.
http://www.adrianflux.co.uk/search/faqs/
There is only one reason for the existence of either voluntary or compulsory excess - it is discourage the insured person from claiming for minor damage, such as broken windscreen mirrors.
If you are involved in an accident and it is your fault but there is no damage to your own vehicle but you have caused damage to another vehicle, property (such as a lamp post), or person you will not pay the excess, even if the claim is £1m.