l wanna buy a wetsuit for surfing in Cornwall, since now my job has changed i'm on more flexible hours so will be able to surf in the evenings.
I've not had one before and need to know what sort of thickness would work now AND in the summer to save me getting 2 suits, also what is there to look out for or any other advice?

If u get a 4/3 mil, That is classed as a spring & fall suit. You will get a little over heated in the middle of summer,But u can always let water in to the suit via the neck. In the middle of winter, say feb when the sea is at its coldest, u wont be able to stay in as long as u would with a 5 mil suit.
Having said that if u keep paddeling round u will stay warm.
Also check out titanium lined suits. as a rule u can loose a mil of rubber & stay warm.
Dont go cheap spend a little more & get a quality one. l have a tailor made SNUGG & they last me 3 to 4 winters.

Been here & spent a lot of time looking into it ! The water is really cold now & will take some time to warm up, even when the sun comes. Anything less than 5mm on ur body & u will be cold. Do not get to into thinking u can bolster with ''thermal rash-vests''. More layers means more water going through & does not always make u warmer. A 5mm body, 3mm arms & legs will do all year round. That is what l settled for & have stuck with for 12 years now. If they do get too hot in the summer, just pull open the neck & u will soon cool. If u really think this is over the top incase we get that ''heat-wave'' in the summer, you're better buying a thermal rash vest & wearing that with a pair of boardies if(and when) the heatwave comes, than buying a thin wetsuit now & need ing a load of extra gear because you're cold - u will end up not going in at all !!

Wetsuits r made for different temps. Standard summer long suit is a 3/2mm, in So. CA that is temps from about 60 deg. F to 66 degrees F. When the temp spikes u can always open it up to cool off, or if sunny just use board shorts. The other thing to keep in mind is that not all wetsuits r created equal. The top of the line O'Neal Psycho I l 3/2mm may be warmer than some 4/3 suites. When the temp drops to below 60 then people start adding booties & hats. O'Neal has a new suite out that allows a zip on hoodie attachment. A 4/3 with booties will keep u warm in the below 60 temps. l checked the temps in ur area & it appears to be 54. So a 4/3mm with booties would keep u warm. It also depends on ur BMI, (body mass index), if u have some of ur own insulation then u will be warm, but if you're really skinny u will want to 'dress warm'. You can always add a 'hat' on colder days & start removing the hat & the booties as the temp increases. However if the sun is out & the temps in the 65 degree range in the spring/fall u may get hot in a 4/3. You may want to determine the avg water temp for each month & then decide what type of wetsuit would give u the most months coverage. The amount of wind & sun will also make a difference. If it is cloudy & windy it will be a lot colder than sunny & still, l am guessing you're more in the cloudy windy areas? Talk to the locals at a surf shop & get their input, they will know best. Ask them what they use.

Jet Pilot and Body Glove are the two best suits for surfing and water skiing. The company 800 number will put you in touch with people to answer all of your questions and then ship what you need. A 2 mil suit is flexible, warm, ---best used with boots or surf shoes in cooler waters. Get what you need by asking these companies. Hang loose and have fun