Sorry, I meant buy the tips you want (as per your link) from the place you intend to fit. You're likely to get a lower overall prive for supply and fit compared with just fitting something you've brought along.
Also, if anything goes wrong or the part is wrong etc etc - it's on the exhaust place to sort out rather than you this way
Magic77 does suggest a good alternative to just replacing the tips.. (full replacement backbox) but AFAIK you're looking at a good £350 or so fitted for a full replacement backbox rather than the £80 I mentioned to get mine done as per the above pics
Hope that clears it up.