THERE are only four weeks until the end of the tax year, so investors who want to use up their annual personal equity plan allowance will have to move quickly.

If you have already taken up your general allowance, have you considered investing in a single company PEP? For investors holding existing shares, it may be a good idea to move them into this type of vehicle.

Single company PEPs are often overlooked. Investors tend to concentrate on using their general allowance which enables them to invest up to #6000 in unit trusts, investment trusts or shares.

Naturally, it is important to establish a broadly based equity portfolio before branching out into single company PEPs, which are higher risk. But as PEP advisor Best Investments points out: ``If you have an equity portfolio of #30,000 or more, it is silly not to take advantage of this further tax concession.''

Single company PEPs also give you an opportunity to place existing shareholdings within a tax shelter. While holdings cannot be transferred directly, plan managers can sell them and buy them back for you within their PEPs. For anyone about to ``bed and breakfast'' their shares to reduce their potential capital gains tax bill, it is a simple step to ``bed and PEP'' them instead.

If you are starting from scratch, there are three main types of single company plans to consider:

q Corporate single company PEPs: An increasing number of UK companies are offering shareholders the facility to place their holdings within a PEP. Employees who acquire shares through share option schemes can also use one if they want to continue to hold the shares. The Bank of Scotland runs around 20 plans on behalf of companies such as Guinness, Scottish & Newcastle and ScottishPower.

The main advantage of these company sponsored plans is that they are low cost. Dealing charges are typically 0.25% and the annual management fee is 0.5%. The drawback is that you are restricted to just one share. If you want to switch, you will have to transfer your plan.

q Self-select single company PEPs: If you want the freedom to buy and sell different shares each year and you enjoy monitoring companies and backing your own judgment, then consider a self-select plan. These are offered mainly by stockbrokers, including telephone execution-only brokers such as Sharelink.

But watch out that costs do not exceed the tax benefits. Sharelink, for example, has fixed quarterly charges of #10 per stock. Its dealing commission is 1.5%, subject to a minimum charge of #20 and a maximum of #37.50. Some general plan managers, such as Alliance Trust and Perpetual, will also allow you to top up with a single company PEP, though the choice of shares may be restricted.

q Managed single company PEPs: Many investors may feel they have neither the time nor the expertise to select their own shares each year and will, therefore, prefer to hand the job to a professional manager who can make the choice for them. After making the initial selection, the manager will also keep a watching brief over shares held and will change them in future years if necessary.

Besides being offered by stockbrokers, managed plans are also available from fund managers such as HSBC and Fidelity. During this tax year Fidelity has been buying shares in the Prudential for those who want income and Glaxo for growth investors.

A more novel approach is provided by London financial group Johnson Fry. It's Hy1 PEP is based on the automatic share selection system developed by US investment guru Michael O'Higgins. This chooses the 10 FT-30 index shares with the highest dividend yield and then homes in on the second lowest priced of these.

Historically, the scheme has out-performed the All-Share index, though the managers stress that it is not infallible. The share currently being chosen is Hanson.

For regular investors Johnson Fry now offers a Hy1 portfolio which contains up to five shares. It will also accept transfers of existing single company PEPs if you are unhappy with what you have got.

The main danger of single company PEPs, unless you choose a managed scheme, is becoming over-exposed to one company. This is most likely to happen if you choose the corporate route. So be sure to resist the temptation to buy the same share each year, however well the company may be performing. Otherwise you will increase your risk because a large part of your investment will depend on the fortunes of just one organisation.

It may also limit your choices in the future. If a plan manager amalgamates more than one year's holdings in one plan for administrative convenience, you will not be able to split the holding in future. So even if you decide to make a switch, you will be forced to invest the whole amount again in just one share.