I am currently using a PC100 m/b with onboard AV and an extremely slow cpu. I intend to upgrade both. I know the logical step is to go with no onboard AV and have decided this already. However with amount of choice regarding m/b and cpu I would welcome any advice or opinions on athlon vs intel and/or reliable best price suppliers in the NE of england or mail order! I do have a budget so no silly suggestions please. Please feel free to email me personally. thankyou all.

You don't mention what motherboard you have at the moment, so we can't tell if you might want to keep it. You also don't mention what you use your PC for or how much money you have to spend.
The cheapest option is a Socket 370 board with a Celeron chip. This won't cost more than £150 and perhaps less if you shop around. Trouble is that this runs at 66MHz memory bus. Many people, myself included, overclock to 75MHz and also thereby squeeze some more performance out of the processor (which is clock locked).
At home I've got a 500MHz machine running at 567MHz with a 75MHz memory bus (and slightly bumped up PCI bus too). It cost around £130 to upgrade. Considering that this is only around 200MHz off the fastest PCs available at the moment then it's not bad going!
The choice of motherboard is very important though - Abit motherboards are particularly overclocking friendly. Do a search on the Internet - there is loads of info out there about it.

Thanks keir the m/b I have is ss7 m571lmr pc100 with a cyrix 333 and onboard AV max fsb 75MHz which is very slow and the bios only offers me a max of 300MHz with multiples thus vslow when everyhing is taken into account.I have about £300-400 to spend but will also need AV cards. What do you think of intel vs athlon????

Sorry for not responding sooner. I thought this thread was dead.
Athlons are clearly superior in terms of performance and cost. There used to be two complaints against AMD (who make Ahtlons): that the floating point performance on their processors was rubbish (bad for 3D games), and that investing in their product line meant deviating from Intel who would always be around and able to deliver, whereas AMD might not.
But both are quashed nowadays. Ahtlon has BETTER FP performance than equivalent Intel processors. And AMD are clearly a huge established company who will be around for a couple of years (touch wood), so investing in a motherboard to take their processor is no bad thing. There will probably always be a processor to upgrade to when yours becomes out of date.
However, the speed at which things progress nowadays means that you generally need a new motherboard if you replace your processor anyway.
Go for it, I reckon.