K=1200mm minimum distance from another flue terminal so just about OK but below is from Part J Building Reg.
Outlets from flues
3.23 Outlets from flues should be so situated externally as to allow the dispersal of products of combustion and, if a balanced flue, the intake of air. A way of meeting this requirement would be to locate flue outlets as shown in Diagram 34 and Diagram 35.
Note: The plume of wet flue products from condensing boilers, positioned in accordance with the safety distances set out in Diagram 34, can sometimes be considered a nuisance for neighbouring properties. Whilst this nuisance is not considered to be within the scope of building regulations, such installations could be considered as a ‘Statutory Nuisance’ as set out in the Environmental Protection Act. As such installers may wish to adopt the guidance in Chapter 6 of the Guide to Condensing Boiler Installation Assessment Procedure for Dwellings
Care may also need to be taken to locate flue outlets away from parts of the building that may be damaged by frequent wetting.
I would suggest a plume kit Jay, discharging products of combustion over the building boundary into someone else property is never a good idea & if wanted to get snotty they could make you alter it.
http://webarchive.nationalarchives....l.gov.uk/uploads/br/br_pdf_ptl_condboiler.pdf
Section 6 suggests 2.5 Meters to boundary !!!