Are they the K30 for £72? They don't specify a weight limit, but they are a conventional spoked wheel so maybe the tension is a bit slack as is often the case with new wheels. Find a decent LBS and ask them to re-tension it if you think chasing Merlin is too much trouble.
No, that's not the problem.
Wheel stiffness does not go up with spoke tension. Steel (spokes) is made from a Hookean material (it follows Hook's law). Basically, it goes like this. If you have a steel spring and you load it with 1KG, it stretches say, 1cm. If the material obey's Hooke's law, the next 1KG will produce another 1cm of stretch and so on, until it reaches the limit of it's elasticity. There things get a bit strange but let's ignore that and go back to spokes.
Being steel, a kg of tension on them gives you x-amount of flex per given applied force. Applying another kg of tension, will result in the same deflection for a given applied force again.
There are ways to make wheels stiffer but they can't be bolted on afterwards. You have some choices:
1) Use thicker spokes.
2) Use more spokes
3) Use a stiffer rim
4) Combine the above.
My guess is that that particular wheel has too few spokes, but I haven't seen it, so it is just a guess. Nowadays thin box section rims are out of fashion, hence my suspicious on the spokes. I don't suspect too-thin spokes because those require high skill from the wheelbuilder and I won't expect that company to throw that type of skill at relatively cheap wheels.
