I've no complaints with my D-Boys M4 SIR, which should have been 300 quid, but was discounted to only 180 quid when I bought it (you can get a sportline plastic receiver version for less - I think about 100ish - but mine's the metal version and it's fairly easy to find it discounted to nearer 200 quid in price). Like all airsoft M4s, technically it is not full metal, because there are non-metal bits such as the fore grip, pistol grip and bits of the collapsible stock, but that's true of the real M4 too.
Apart from adding a tac light with a pressure switch on the front tactical grip, I've left it completely stock. It's very accurate and has good range. I don't even use optics with it, I just flip the rear sight up and leave the front sight down and line up the front sight gap with the rear sight pipper, which is quick and easy to do with it on a three point sling carrying it in either high or low ready. Three taps on semi auto using that sighting method generally does the job both close in at CQB ranges and in woodlands at longer ranges. Comes with a very reliable 300 round high cap mag as standard, so that's nearly good for all day providing you aren't the spray and pray type, although I do have several 160 round mags for it too.
D-Boys weapons appear to be more common in the States on skirmish sites than in the UK from what I can gather - I don't see many people with them - but I think that's more a case or people going with the crowd and choosing more common choices such as CYMA and G&G rather than because of any reliability issues - and since it is a fairly bog standard M4 under those SIR rails, there's no issue with parts compatibility, i.e., it will take standard M4 mags and other standard M4 parts. Since your budget is 400 quid, if you got one like mine, that'd give you 200 quid to go mental on upgrades if you wanted a fancier barrel or some freaky sight or grips or whatever, since the SIR has a large amount of rail space.
In terms of reliability, it has been pretty much flawless. I do take spare guns with me to skirmishes, but I've never had reason to switch from using my D-Boys M4. It's tough too; I'm one of those woodland types who is not afraid to crawl through shite and dive in the dirt, or hug walls in CQB where it is easy to ding your flash suppressor into a wall, and the D-Boys is fine with that kind of handling, a quick wipe with a cloth at the end of the day and it looks good as new.