Can anyone point me to a good source of information regading the effects of solar heating
on pipework?
I am reviewing an external piperack of some 300m long which is carrying all manner of tubes/pipes and ducts.

Due to the long length of the rack, a small delta t of the pipe work will result in worryingly large expansions.

Pipes include 14" duct(2mm wall)
and 4" shd 40 pipework.

Any advice is appreciated.

Whats the problem?? All materials expand and contract based on the current temperature being considered versus the installation temperature the system became "fixed" at, whether these delta temps are due to solar gain or the temperature of the fluid they contain.

And yes long runs may require some consideration for these minor temperatur variations. One school of thought is to provide flexibility as required the other school tries to provide enouhg guides so as to be able to take the thermal load into a pure compressive load with no unstable column buckling.

I am trying to establish what temperature the sun
may heat the pipework to.
I am lead to believe that 60c is a norm, however i have no prove of this.
As large Expansions will have a detrimental impact on the project, i need proofs for why the
Pipework may reach elevated temperatures due to solar heating.

140F is commonly used, however if you have doubts go into your Marks Mechanical engineers handbook or whatever else you may prefer and calculate the gain possible for the latitude the project is being built at... who know 140F may be far too low or far too high.