As I've been getting a few questions in from people and can't get along tonight please see below a couple of comments on spikes / trail shoes from me before "spike night" which might help - I'm sure the team that are there on the night will be equally helpful.
Generally spikes come in two types for distance / XC running.
Middle distance spikes are generally very "aggressive" and will have a plastic plate on the underside. These will really struggle badly on even hard surfaces and aren't generally recommended.
Sample models: Nike Victory, Saucony Endorphin MD
Long distance spikes are again designed for track running but aren't anywhere near as aggressive. They may have a small plastic section on the forefoot. They will have limited grip and very little cushioining around the heels and it will be a harder material generally than rubber. These are lighter than XC spikes generally and slightly tougher on the achilles.They have very lightweight / flimsy uppers which are liable to rip quite easily.
Sample models: Nike Matumo, Saucony Endorphin LD
XC spikes tend to have an entirely rubber sole on the underside which often has some designs built into it to help grip better and more cushioning along the length of the spike - this makes them less hard work on the calves and achilles. The uppers are tough and hard wearing. The shoes are generally a bit heavier than the LD spikes.
Sample models: Nike Victory XC, Nike Zoom Rival XC, Saucony Kilkenny
The alternative to spikes is trail shoes. Generally for XC you will want to go for more of a fell shoes than a XC shoes. Most fell shoes are actually pretty light-weight and are often built slightly less aggressively than an equivalent spike with more of a heel-drop (Bare Grip excepted). If you have struggled with achilles problems I would recommend a pair of these (but not the Bare Grip).
Sample models: Walsh PB, Inov8 X-talon 190 / 210, Inov8 Bare Grip 200
As regards which one suits you best - trail shoes are suitable for a variety of runs and distances. You can wear your spikes on track (regardless of whether they are XC or track spikes - for training I would actually go with XC spikes as the rubber sole reduces the injury risk) and this isn't a bad idea - particularly during the winter when it can get icy.
For XC races - in moderate conditions with a mix of trail and mud I think the benefit risk/reward of wearing slightly faster / better gripping spikes doesn't come ahead of good trail shoes with less injury risk. Most of the LEAGUE XCs are in fairly good conditions on Wimbledon Common - you can definitely get around with an aggressive pair of trail shoes.
For the more muddier races (Surrey XC, Southern XC, National XC and anything in Lloyd Park) - spikes come in more use because you can install long spikes which give you really good grip even in deep mud... for the absolute worst courses I have an inclination that when the mud is so deep that you can't even reach the solid ground with your longest spikes that something like the Bare Grip or X-talon with grip all along the length of the shoe is more useful.
Regarding spike length - you should get a variety of spike lengths. Generally 6mm is suitable for short grass courses (e.g. a golf course) with lots of trail. 9mm is standard length for most courses. 12mm are suitable when the course is muddy and entirely on grass / mud. 15mm are monster spikes and only suitable really for the Southern / National XC. You may want to get a variety.
Regarding caring for your spikes - you will want to put a dab of vaseline on each spike as you insert it using the spike key. This will help it come out more easily later on. Take care to have the spike key that's right for your spikes (the actual spikes not the shoes) as otherwise you will wear away the thread on the spike and not be able to get it out properly.
After each race you should handwash your spikes and remove the spikes from the bottom of the shoe - you strongly risk them rusting in otherwise.
Hope that helps - chuck up anymore questions.