Paragraph 11.1h of the model Financial Regulations states:-
"where the value is below £3,000 and above £100 the Clerk or RFO shall strive to obtain 3 estimates." A formal tendering process is not required and would deter providers from quoting, due to the amount of work required and the low likelihood of securing the contract.
There are a number of specialist providers of parish council websites and a Google search will find them. I've just looked at one example offering sites from £399 to £699, depending on scale and complexity. If you already have a website and are seeking only minor modification, you could approach a small local web design business to do this for you.
For a service such as this, where the provider does not need to be local, the lack of competitive quotes/estimates is not an acceptable excuse for committing to an over-priced quote.