Have a friend that uses his 598s at work. He's got an office...so it works for him. They are nice headphones but I like my 558s a little better -- 2/3rds the price and a more balanced sound signature for my tastes. Regardless, the Senn 5X8 line is a great value across the board. Right now at £149, the 598's seem like a very good value. Can't go wrong with either...just wonder if they (open cans) would work for you in your office environment.
Some others to consider from low £ to high £
- Creative Aurvana Live or CALs - closed cans; do well across genres, really great value at £52.83 on Amazon
- UE6000 - closed cans, previously on Tyll's Innerfidelity Wall of Fame (http://www.innerfidelity.com/content/surprisingly-good-logitec-ue6000-and-ue9000), also a great value at £78.74
- ATH-M50x - closed cans; updated version of a classic (although cliche) recommendation for £121 -- also on Tyll's WOF (http://www.innerfidelity.com/content/legend-continues-audio-technica-ath-m50x). TBH, I'd recommend the KNS8400 at £99 over the original M50 but it sounds like they fixed some of the M50's issues in the M50x
- Beyer DT770 - closed cans; 32, 80, or 250 Ohms; not as balanced a signature as 880s...kinda audiophile basshead (great if you like Hip-Hop, EDM, Dance); less cash than 880s, range £100-130
For open cans, a few around £100 might be worth a look; the Grado SR60s or SR80s (£79-99)...you either love them or you hate them. I like the 60s more...better bass/less treble. Also, the Yamaha HPH200 (£90) come to mind--haven't heard them but would like to based on reviews. Also, DT990s are £101.
That said, since you like AKGs, it makes sense to give those serious consideration.
FWIW, I'd echo the concerns about the 550s; listened to them a few times and just didn't enjoy them -- fit and sound were a little wonky for me.
However, I really like my Q701s but they might be outside your budget. If so, then how about K612 Pros at £121?