Upgrade my Rear Cassette from 12/25t to 11/28t I have a Specialized Secteur 8-speed road bike with a Shimano HG850 12/25t rear cassette running on a KMX Z type narrow chain my crankset is a 52/42/30, I have done about 2,000 miles so it's probably time to change my chain so why not the cassette as well.

I want to achieve a bit more top end to stop me spinning out without to a 10 speed and having to change the shifters. I have had a look on Sheldon Brown I and have found several combinations of new rear cassette from an 11/28t, 12/25t, 12/21t to a 13/26t

I have found several 8-speed cassette's with these ranges. Shimano (HyperGlide) HG-70 , HG-50 and SRAM (PowerGlide) PG830 ,PG850

However the HG-70-I is a new one on me I think it's an IG (Integrated Gear) I believe that the sprockets are thinner and closer together? Hhere is what Shimano website tells me.

All current HG cassette cogs are specially profiled to be HG or IG chain compatible.
Any Shimano IG chain will work with any HG cassette. If you use a HG chain with a 7-speed IG cassette (CS-IG50 and CS-IG60) the rear derailleur adjustment becomes more critical.
One significant compatibility issue is always to avoid using a HG chain with an IG-only crank set. Why? The chain will release from the large (outer) chain rings too easily while in the larger rear cog. It may be possible that the chain drops off the outer ring without shifting the lever, when under full power.

Also I have been told that I might need a longer rear mech to handle larger than 25t on the cassette not that I need to go any higher than 28t as I am running a triple chain set. I am not sure if my rear mech is a medium length if it helps my rear derailleur is a Shimano RD-2300 like the front.

Any help would be appreciated I am sure that I can do this for about £30 (£20 cassette and £10 chain) but I want to be sure.
Usually, the up to 26t is more so a guideline than a hard limit as is the chain wrap capacity. I'd say try the 11/28t - it will probably work, and if it doesn't work you can always put a new long cage Shimano derailleur (such as a deore one - the cable issues dont matter for road vs mountain rd's from Shimano, or a Sora if you want to stick with the road group). Of course, if you don't shift properly, there is a higher risk of things not working the way they should. 

Though, you probably just need to spin faster instead of higher gearing (use Sheldon Brown's gearing calculator to see the differences): http://sheldonbrown.com/gears/