Hi everyone, thanks for the replies...my responses are below.

In reply to a post by MrSaffron:

How far apart are the two routers?

They're next to each other (they have to be, because they're connected via ethernet cable).

In reply to a post by Stevenage_Neil:

In reply to a post by Oldjim:

How am I supposed to know

Perhaps you know what channels they are on then?

The PlusNet box is on Auto channel 6. The Apple is on automatic.

In reply to a post by mHm:

In reply to a post by urban4691:

master socket - cable - PlusNet box - cable - Apple Time Capsule
...
...
How is that possible? And how can I sort out this strange discrepancy?

Neither "PlusNet Box" nor "Apple Time Capsule" are anywhere near precise enough names to determine what hardware you have there. Without, nobody will be able to give a qualified answer.

It's a Technicolor TG582n and Time Capsule 802.11n (1st Gen) using firmware 7.6.4.

In reply to a post by urban4691:

Hi everyone, thanks for the replies...my responses are below.

In reply to a post by MrSaffron:

How far apart are the two routers?

They're next to each other (they have to be, because they're connected via ethernet cable).

The maximum length of an Ethernet connection, with no problems, is 100 metres .

In reply to a post by RobertoS:

In reply to a post by urban4691:

Hi everyone, thanks for the replies...my responses are below.

In reply to a post by MrSaffron:

How far apart are the two routers?

They're next to each other (they have to be, because they're connected via ethernet cable).

The maximum length of an Ethernet connection, with no problems, is 100 metres .

I just moved the boxes as far away as they can go and...no change in the Time Capsule's wifi speed. But the PlusNet box...has gone from 2MBPS to 17MBPS! Problem solved, I think. Although: I just tried a Skype call, and my side of the connection kept breaking up really, really badly (to the extent that it's better on Vodafone 3G).