Simon, the 17” Monza’s on the mk6 GTI are (I think) 7.5J x 17 ET51 and, the OEM 18” Austin’s are 7.5J x 18 with (as you’ve said) an offset of ET49.
Both 17” Monza’s and genuine 18” Austin’s will have the same bolt pattern (PCD) of 5x112 and a centre bore of 57.1 so the Austin’s should bolt straight on. The Austin’s having a slightly more aggressive offset (ET49 compared to your existing wheels ET51) means the inner rim will be 2mm further away from the suspension strut and the face will protrude outwards a further 2mm.
Although I think the Austin’s are a nice design of wheel, I’m personally not a fan of diamond cut wheels, as they’re often prone to white worm corrosion at 2-3 years old and require a refurb to restore the finish back to original. A good quality diamond cut refurbishment costs in the region of £90+VAT per wheel. Also bear in mind that diamond cut alloys can usually only be refurbished twice as the process involves re-cutting the face of the wheel by removing a layer of metal on a diamond cutting lathe. More than two refurbs and the structural integrity of the wheel can be compromised.
If you’re thinking of buying used Austin’s, be aware that there a lots of replicas out there, some may not be particularly good quality and may not have been subjected to the same stringent safety and quality tests that OEM wheels are. Replica wheels should be advertised as ‘Austin style’ alloys if they’re not the genuine article. Some of the replicas will also have a more aggressive offset than the OEM wheels. The originals should have the VW logo and spec of the wheel (7.5J x 18 ET49) stamped into the reverse side of the wheel spokes, along with the VW part no. (5G0601025AS).
Good luck in your search.