Can someone advise me on the width of pipe needed for a gas boiler?

Hire a professional for the job. That way you know it's done right. Whatever he charges, it's cheaper than a fire.

The size of the gas pipe is determined by the number of gas appliances, the KW input of all the gas appliances, the distance from the meter to each of the appliances and the number of bends and tee connections needed to complete the pipework for all the appliances. Some of the gas run might even need to be run in 28mm if the length is over a certain distance.

The technical spec for the boiler will state the size of pipe that needs to be connected at the actual boiler. The rest needs to be worked out subject to load and route.

If you are referring to the pipe to supply Gas to the Boiler, then it has to be a 22mm pipe all the way from the gas meter to the boiler, except for the last 6 feet of pipe, which can be reduced to 15 mm pipe. This ensures that when other gas appliances are being used, the boiler is not starved of gas.

Your new boiler will come with installation directives including the size of supply gas pipe. These directives will likely match any legal requirements but remember that manufacturer's requirements trump the code requirements if different.

It depends on your gas pressure and the demand of the boiler, your gas engineer should work out the requirement.

The legal requirement is to have the installation done by a registered and qualified gas fitter. That is the LAW, no-one else is permitted to do the work. They will tell you what size of pipe they will be fitting. (usually 22mm)

http://www.gassaferegister.co.uk/

It will probably depend on the type of boiler. I would check with the manufacturer of the boiler you are planning to have fitted and then use the engineer who got it right.

ask a gas safe engineer

22 mm minimum