Does the boiler come under the PSSR?
Have a squint at this:
http://www.hse.gov.uk/pubns/books/l122.htm

If it does then quite possibly, it's been a while since I've done any pressure systems stuff, but IIRC, the WSE must be prepared by someone @ IEng or above, thus realistically IEng or CEng.

The basic requirement of the PSSR

The Regulations are concerned with steam at any pressure, gases which exert
a pressure in excess of 0.5 bar above atmospheric pressure and fluids which may
be mixtures of liquids, gases and vapours where the gas or vapour phase may
exert a pressure in excess of 0.5 bar above atmospheric pressure.

This is a direct quote from the ACOP.

So does the boiler contain a relevant fluid? See Figure 1 in the ACOP

(a) steam;
(b) any fluid or mixture of fluids which is at a pressure greater than 0.5 bar
above atmospheric pressure, and which fluid or mixture of fluids is—
(i) a gas, or
(ii) a liquid which would have a vapour pressure greater than 0.5 bar
above atmospheric pressure when in equilibrium with its vapour at
either the actual temperature of the liquid or 17.5 degrees Celsius;
or
(c) a gas dissolved under pressure in a solvent contained in a porous
substance at ambient temperature and which could be released from the
solvent without the application of heat;

Water Pressure is 6 bar
Water content 8 litres
Gas Pressure 25 mbar

Taking the above information from the spec of the boiler....PPSR would not apply?

Provided your pressure units are correct - mbar, then this is much less than 1/2 bar.

1bar is about 1 atmosphere pressure = 14.5psi (approx)

25mbar = 0.025bar.

However, if you read the PSSR, any pressure vessel containing steam at any pressure is covered.
I am guessing that your reference to gas pressure is that it is a gas fired boiler?
The water pressure could be a maximum inlet pressure of 6 bar?
This spec is not quite enough to go on.

If the vessel holds steam under pressure, then it comes under PSSR.

Colossians,

What is the make & model of the boiler?

(Can anyone tell I'm in the office with loads to do, so I'm procrastinating!!!) ;)

Can't find the manual on the Hamworthy site, but to my mind it's a heating boiler yes?
For heating hot water or for building heating.
In which case it comes under PSSR as it will contain steam at pressure, any pressure.
Your company insurer will probably be inspecting this under PSSR for you anyway effectively as they will be inspecting it for your insurance policy reasons.

See Figure 1 of L122 ACoP

http://www.hse.gov.uk/pubns/priced/l122.pdf

There are also useful FAQs on HSE website.

That's a good point actually Ian, the only Hamworthy boilers I've ever worked with have been steam generating.
This one could be simply a water heater like a glorified domestic boiler, rather than a steam generating boiler.