I've just replaced our ancient (15 yr old) Sharp with another Sharp. It's the R353 - a straightforward microwave, no fancy bits and it's fine. There are versions with grills etc. It was £99 in John Lewis. Btw, the old was still in full working order - I had just decided I could see microwaves leaking everywhere and anyway it was ENORMOUS, being so old!!
One big tip - measure your steriliser and make sure it will fit in your new microwave! My next door neighbour had her daughter to visit and popped round to sterilise her granddaughter's bottles because the unit wouldn't fit in her microwave!
I would also say save yourself some money and just get one that defrosts and cooks. If you are like me the only thing I use my microwave for is defrosting meat, heating up beans and jacket potatoes and sterilising bottles. I never ever use the grill facility, nor have ever cooked a cake in it etc! The only thing I like about it is it has a timer!
Oh, I'd say it's worth the money to get one that has a 5 year guarantee, grills, defrosts, times cooking and acts as a conventional convection oven when you want it to. I have a Panasonic (don't know the model, sorry) that does all this and it's brilliant. Especially if I want to do a chicken in the oven at 180c and roast potatoes in the microwave, used as a convection oven, at 220c. Or leave something cooking in the oven and steam veg in it (electric hob, crap, takes ages to warm up) But mine was a present so I didn't have to spend the £500 to get it... I couldn't manage without it.