The requirements form part of Building Regulations Part J, which are to do with protection from heat-producing devices. The relevant document is here.
http://www.planningpo...r/BR_PDF_ADJ_2002.pdf (big document)
On page 43, Diagram 34 shows the acceptable location of outlets in relation to the distance to other parts of the building. The table on the following page annotates the letters on the diagram.
"is too close to adjacent wall" means just that. It will either be in relation to a return wall at the side or the roof of the porch above it.Diagram 39 may be helpful as well if you have the outlet shielded from combustible material. This assumes gas - there is another diagram a few pages further on for oil-fired.
"has no guard" is covered by para 3.25 and 3.26 on page 42. A guard is not obligatory - the text comments about situations when birds or squirrels might try and nest or contact by hand may occur. Presumably the latter here. Probably best to just fit the guard rather than fight the interpretation placed.
Building Regs are not retrospective unless some upgrading has occurred when the latest regulations generally apply - unless it is impossible to do (example can't make a staircase in an old cottage apply to new regs). However they are good practice practice. But presumably you've had some mods done to the boiler (replacement?) as BR Inspectors generally don't go around the town looking for such defects.