A solicitor will charge around £120 for a simple Will. This will be legally correct and fully binding. The solicitor will run through the common provisions and pitfalls that even the "simplest" will can encounter.
You should understand the process of Probate: this is complex and time consuming even for the very simplest of estates. Nothing can be done until this is granted. It could take between six months and a year (or more).
The advantages of using a solicitor is that:
1) there is no problem getting witnesses who are legally appropriate for the role.
2) unless you are 100% certain that your descendants will not argue - there is a legal form of words which will make your wishes for division of your estate clear and enforceable by a court (if necessary).
3) if you want your named beneficiaries to be the sole inheritors and free from "gold grabbing" spouses, and/or second wives/husbands and their children, then you must explain this to the solicitor and he will draft this in binding legal format.
4) he will explain how you should arrange your affairs and financial information (bank accounts, investments etc) so that your executors can easily gather your estate together. You won't be doing anyone any favours if you have money in a "forgotten" account that nobody knows about or can get into.
5) he will explain the implications of taxation and the legal priorities for disposal of your estate. When Probate is granted the HMRC must be paid first, next to be paid is your local authority taxes, then your commercial debts must be paid (including paying off the mortgage if you have one). Finally your legacies are distributed from the "residue" of your estate.
6) the location of the "legal" version of the Will will be in the safe keeping of the solicitor (there is normally no extra charge for this) and this will be known to your executors because you will (as a sensible person) keep an copy (not usually legally binding) in a known location in your house. That keeps the legally binding Will safe from tampering (and hence challenge).
7) the solicitor will explain the implications and advantages of a legally binding "Enduring Power of Attorney". This is what used to be called a "living will" and deals with management of your estate if you become incapable of doing it yourself. It's a scary subject (thanks to Hollywood dramas bout greedy relatives) but it's something that you really should consider ... Alzheimer's or comas etc can strike Anybody.
You can buy a "kit" from WH Smith for much less. But it will be "generic" and you MUST follow the instructions precisely or it will be invalid. You must be incredibly clear and precise how you want your estate to be distributed. Considering the tax and debt implications you would cause problems if you specified £XXX to each party and the estate was unable to meet that wish. You must also make arrangements for the safe (and tamper-free) storage of the Will and that the executors know where it is. This may not be as simple (or cheap if you us a bank box) as you may think when you really sit down and consider it. Remember - copies are not legally binding.
You can also use a professional Will writer (about £60-£70). These advertise in local papers etc. but there is no legal training or registration for these so be very careful who you choose.
You will still need two independent witnesses (their name doesn't matter). It is also legal for an executor to also be a beneficiary.
On balance I strongly recommend using a solicitor. If you care about your beneficiaries and executors this is the only way to clarify properly what your wishes are, and to make the disposal of (even the "simplest") estate as pain free as possible. It may seem a lot of money but it is SOOOO worth it for your peace of mind (and your family).