Firstly Robsterno1, don't get drawn into replies with those that choose to post less than helpful responses. This has happened to me before in the past when seeking, as is in your case, some genuine helpful advice. Annoying though it is, it just offers 'grist to the mill', so if on here again just refrain and ignore. That's what I do and I'm Gas Safe registered and G3 qualified.
That being said you may wish to consider the more helpful responses that were given. Notably, Joule over some brand that no-one has heard of. In my business I offer my customers several cost options ranging from entry level RM's up to Joule or Gledhill cylinders. I also offer them the option of a dual coil cylinder which will facilitate a very fast recovery time when both coils connected to a single heat source. Where, the secondary coil can be connected to an ASHP (Air Source Heat Pump) or Solar Thermal IF the customer should so desire to install one at a later stage. Obviously, a single coil cylinder will be cheaper to purchase than a dual coil one.
I agree with the opinion expressed about staying away from enamelled cylinders. Obsolete technology in my opinion that should be avoided.
In general, with regards to the domestic unvented cylinder market, the more expensive the cylinder is the more likely it has been manufactured from a superior grade of Duplex stainless steel. There exists three main grades of Duplex S/S: Lean, Reference (sometimes known as Standard) and Super, respectively. All of which have there specific commercial applications and now more widely employed by cylinder manufactures. I suspect, but can not be clear as no manufacturer is willing to divulge this commercially sensitive information when I made my own enquiry some time ago, that the cheaper cylinders are made from Lean grade Duplex and the more expensive from Reference (or standard) grade Duplex. The later having a higher Molybdenum content which in turn gives a greater overall resistance to chloride stress corrosion cracking. Thus the difference in manufacture warranties and price point.
In addition, as helpful contributors have remarked, water pressure is not the only consideration in this instance when upgrading to an unvented cylinder. The flow rate (volume of water/unit time) is important too and is directly related to the cross sectional area of your incoming mains water supply pipe. Your friend, should undertake a dynamic pressure test and flow rate at the nearest unrestricted draw off point to the internal stopcock to quantify this. Rule of thumb is approx. 20L/min at 2bar. Consequently, will an upgrade to your mains water supply need to be undertaken? Inside the house? Outside the house? Or both?
It would not be unreasonable to ask your friend to detail a schematic of that which is proposed and then if you wished for it to be reviewed by useful contributors on here I'm sure you will get some helpful feedback.
Other points to note, when converting over your domestic hot water set up, is the potential risk for component failure due to the higher standing water pressure as a consequence of the hot and cold services now being pressurised to a maximum of 3bar. Where as before, notwithstanding anything in your current set up that was served directly off of the mains, would have a standing pressure of approximately 0.5bar...dependant of course on the height of the cold water storage tank in your loft above the outlets it serves.
In addition, and a point I posted on not that long ago, is the need for the plumber to ensure a balanced hot and cold draw off through out the house. Therefore, ask him what aspects of the existing plumbing will remain and what will change to being served by a balanced supply.
Lastly, consider location. As it is proposed to install a mains fed cylinder this can, within reason, be at any location in the house, notwithstanding D2 termination requirements. This being a term that if he's G3 registered he will be able to explain and discuss with you.
These are all basic design elements that through good practice and communication with your customer can be discussed and decided upon before the commencement of works. Informed choice!
Hope this helps in some way.
