Help selling an Internet site and business

Hi all. 5 years ago i set-up and internet business in the UK. The company supplies services (industrial) to the public and trade sector. Initially the company supplied the services from a small factory. The company has grown to be the market leader in it's field and it's brand is almost a household name among it's market. It has more enormous rankings on search engines and for what is a small cottage business receives huge traffic that is well suited to the many services the company provides. We also have a strong offline presence, we attend shows and do numerous localized and nationalized marketing hits. The company offers it's service across the whole of Europe.

After 18 months it made more sense to pass our work on to a larger factory that could handle the sheer volume we have.

That factory would now like to buy our business. Our work input has been so large they have had to employ more staff and expand their floor-space to compensate.

They have proposed a deal:

1. they buy the website and the brand 2. they pay of it over a 3-5 year payment plan 3. I stay on as a paid consultant to help develop into further markets and maintain postion. 4. they want a guarantee that if the companies internet presence was to slide - effectively in their words they can seize paying

Point 3 causes me concern. Ideally i would like to sell it for cash, perhaps over a few payments. I'm happy to give them 3 years. I experience with them and I think the merging of our brand into their factory will be tremendous for consumers and for them. What i dont like is having to gaurantee performance of the brand over a long period. What or how is this usually dealt with.

Lastly. The value. We receive circa 1.25 million GBP of inquiries per year. Part of the proposition proposal demonstrates that the turnover of the company can be greatly increased. Can potential such as this be valued?

Thanks for your help