Have you actually got a quote from the main dealer at all? Just give them a call to ask, what can happen?
I often think main dealers are slagged off too much personally, my local main dealer has been servicing my Leon Cupra for the last 3 years, first 2 lots of 12 month serving was £150, they fixed my heated seat for £250 (exceptional value, they spent 5 hours on the car that day) and my most recent 40K service was just over £200.
I admit your mileage is much higher, but mine needs its first cambelt to be changed its the same job after all really, they quoted me £
290 all in, VAT, tensioners, water pump the lot. I think thats a fair price, I will be booking it in today.